hal: Add MCU-Driver-HAL and toolchain with FetchContent

Use FetchContent to obtain MCU-Driver-HAL and iotsdk-toolchains. This
allows both the SDK itself as well as HAL implementations to depend on
MCU-Driver-HAL without adding duplicate conflicting CMake targets, as
FetchContent gracefully handles multiple declaration of the same module,
unlike with git submodules.

Signed-off-by: Wilfried Chauveau <wilfried.chauveau@arm.com>
Signed-off-by: Jaeden Amero <jaeden.amero@arm.com>