Add mcu-driver-hal and mcu-driver-reference-platforms-for-arm

The only Arm target currently supported by MCU-Driver-HAL is AN547_MPS3
(Corstone-300).

To build,

    cmake -S . -B _build -GNinja -D VARIANT=FVP
    cmake --build _build

To run,

    # replace <path-to> and <your-choice>
    <path-to>/FVP_Corstone_SSE-300_Ethos-U55 -a _build/_deps/mdh-arm-build/mdh_arm/examples/<your-choice>/*.elf

Temporary workarounds:
* For now, only mcu-driver-reference-platforms-for-arm.git is fetched
  because it pulls in MCU-Driver-HAL as a git submodule. Once the HAL
  migrates away from git submodules, MCU-Driver-HAL should be fetched
  directly and target-specific implementations should be fetched
  according to build target.
* As we do not have proper CMake toolchain files or an MCU selection
  mechanism in place, configurations are hardcoded inside MCU-Driver-HAL
  for now, its subdirectory is added before the top-level IoTSDK project
  is declared.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>