Cache the SDRAM once it is verified (cache=on)

[JohnAmadis]

Summary

dmfmc now caches the SDRAM once it is verified (cache=on, the default).

• Before: the SDRAM window was Normal non-cacheable, non-bufferable (MPU region 6). Every access to it went over the 16-bit FMC bus, whatever the CPU caches. On the STM32F746G-DISCO, filling a 118x101 rounded rectangle took 6.5 ms (~117 cycles per pixel), and a dmview window redraw took 160 to 230 ms.
• After:
  • the window is non-cacheable during bring-up and the uncached check, as before;
  • then the new port function dmfmc_port_cache_sdram() makes it write-back (TEX=001 C=1 B=1);
  • it then checks the SDRAM through the D-cache: 64 KiB of address-dependent patterns are written into the cache, cleaned to the SDRAM, invalidated and read back, in two passes, the second inverted;
  • when that fails, the window is made uncached again and an error is logged.
• Without a data cache (the F4, or the D-cache off), only the attributes are set.
• cache=off keeps the SDRAM uncached.
• Docs: the comment on the MPU region now says why this is safe. The "every word reads back as the last one written" corruption from bring-up coincided with swapped bank target bits (CTB1/CTB2) and too fast an SDCLK for RBURST, both fixed since.

Also in this PR: dmfmc_types.h includes stdint.h, stdbool.h and stddef.h before dmdrvi_ioctl.h. That header uses size_t without stddef.h; the dmdrvi fix is choco-technologies/dmdrvi fix/ioctl-stddef.

Requirements

• choco-technologies/dmlcdtft fix/sync-copied-area should be released together with this PR. With a write-back cache, the area PRESENT copies into the other buffer must be synced.
• The caches are enabled by dmod-boot (choco-technologies/dmod-boot feature/cortex-m7-caches).

Testing

STM32F746G-DISCO, 8 MiB SDRAM, SDCLK 72 MHz, CAS 2, 16-bit bus:

• the check through the cache passed and the window stayed write-back (MPU region 6 RASR 030b002f);
• the board ran the dmview demo, with modules, heaps and framebuffers in the SDRAM, through many minutes of use.

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