mcu-driver-reference-platforms-for-arm directory structure update

This commit updates the reference platform repo to the new
directory structure. Examples / Tests are moved from mdh_arm
to the top-level, and CMSIS is moved from the top-level to
the components directory.

This commit also renames mdh_arm/ to drivers/ but more work
needs to be done to seperate the platform specific parts from
the generic parts in this directory. This will be done later.

Signed-off-by: Bilal Wasim <muhammadbilal.wasim@arm.com>