cmake: Fix dependencies of mdh-shim-drivers

The list of libraries `mdh-shim-drivers` links to is incomplete.
`cmsdk-uart`, which is a PUBLIC dependency, is missing so add it to the
list.

Other dependency, `mdh-arm-platform-common-api`, is unnecessarily
public, so make it PRIVATE.

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