templates: Define MDH_PLATFORM for every template

Templates hosted in this repository are configured for reference
platforms for Arm only. When building one of them, provided by
mcu-driver-reference-platforms-for-arm, it is required to provide a
valid definition of MDH_PLATFORM, and its VARIANT (fpga or fvp).

Add the missing definitions to tests and examples.

Signed-off-by: Filip Jagodzinski <filip.jagodzinski@arm.com>