drivers/devfreq: add missing governor_data field and ondemand declaration

[zzby0]

Summary

The ondemand governor (drivers/devfreq/devfreq_ondemand.c) stores its
private state in dev->governor_data and defines devfreq_ondemand(),
but neither the governor_data field in struct devfreq_s nor the
devfreq_ondemand() declaration were present in
include/nuttx/devfreq.h. As a result, building with
CONFIG_DEVFREQ_GOV_ONDEMAND=y failed to compile.

This gap was introduced during upstreaming of the devfreq framework: the
two definitions originally lived in a downstream commit whose main topic
was a cpufreq refactor, and that commit was not part of the devfreq
upstreaming series. The performance and powersave governors do not use
governor_data, so the problem only surfaces when the ondemand governor
is enabled (which additionally requires !CONFIG_SCHED_CPULOAD_NONE and
is off by default), which is why it was not caught earlier.

This PR:

• adds the governor_data field to struct devfreq_s
• declares devfreq_ondemand() alongside devfreq_performance() and
  devfreq_powersave()

Reported-by: @raiden00pl

Impact

• Fixes a build failure for CONFIG_DEVFREQ_GOV_ONDEMAND=y.
• No impact on existing configurations: governor_data is a new field in
  struct devfreq_s and the declaration is additive; the default build
  (ondemand off) is unchanged.
• No API breakage; no behavioral change to the performance/powersave
  governors.

Testing

Host

• OS: Linux x86_64 (Ubuntu)
• Toolchain: host gcc (sim)
• Base: apache/master

Before this PR — building sim with the ondemand governor enabled
fails to compile devfreq_ondemand.c because struct devfreq_s has no
governor_data member and devfreq_ondemand() is undeclared.

After this PR — clean build:

./tools/configure.sh -l sim:nsh
kconfig-tweak --file .config -e CONFIG_DEVFREQ
kconfig-tweak --file .config -e CONFIG_DEVFREQ_PROCFS
kconfig-tweak --file .config -e CONFIG_DEVFREQ_PROCFS_QOS
kconfig-tweak --file .config -d CONFIG_SCHED_CPULOAD_NONE
kconfig-tweak --file .config -e CONFIG_SCHED_CPULOAD_SYSCLK
kconfig-tweak --file .config -e CONFIG_DEVFREQ_GOV_ONDEMAND
make olddefconfig
make -j

Result:

CC:  devfreq/devfreq_ondemand.c
...
LD:  nuttx

devfreq_ondemand.o is produced and linking succeeds with no errors or warnings.

tools/checkpatch.sh -g apache/master..HEAD passes.

[zzby0]

@xiaoxiang781216
Does devfreq really need "ondemand" governor?

1. Ondemand requires enabling CPU load as a prerequisite.
2. Ondemand is designed specifically for CPUs, not general-purpose devices.
3. Ondemand has not been extensively validated on our products.

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• qemu-armv8a
• qemu-intel64
• rx65n-rsk2mb
• s698pm-dkit
• stm32-nucleo-f103rb

[xiaoxiang781216]

@xiaoxiang781216 Does devfreq really need "ondemand" governor?

1. Ondemand requires enabling CPU load as a prerequisite.
2. Ondemand is designed specifically for CPUs, not general-purpose devices.

user can enable it only for cpu freq scaling.

3. Ondemand has not been extensively validated on our products.