Register RTC with the generic device contract and VFS access fix

[JaxCalder]

Register dmrtc under modules/dmrtc and advance the dmdrvi/dmdevfs gitlinks to the tested generic RTC contract and filesystem access-mode fix. The RTC client uses the shared device contract over VFS and can work with other RTC implementations.

Changes to review and release before integration:

• Define a generic RTC device ioctl contract dmdrvi#29 — public RTC interface and capability semantics.
• Fix driver open modes and mount lifetime during hotplug work dmdevfs#72 — translate VFS RDWR=3 into driver RDWR=4.
• Implement RTC driver with generic dmdrvi contract and STM32F4/F7 ports dmrtc#1 — STM32F4/F7 implementation, generic CLI and tests.
• Register dmrtc calendar driver dmod-registry#16 — package registration after the initial dmrtc release.
• Start device drivers under RTOS and enable RTC dmod-boot#131 — scheduler-backed boot and board preset.

Validation: RTC native ctest 3/3 (12/12 driver steps plus wait/client tests), F4/F7 builds; STM32F746G-DISCO RTC 11/11, dmdrvi 6/6, RNG 6/6. dmdevfs access-mode suite 10/10; two broader dmdevfs tests still SIGSEGV, reproduced against unchanged upstream source and documented in its PR. No dependency version pins or local build URLs are added to production source. The pre-existing dirty registry gitlink is excluded from this commit.