ArmPlatformPkg: add package .DSC file

ArmPlatformPkg: add package .DSC file

Now that we have removed all the cruft from ArmPlatformPkg, add a .DSC
file that builds all the remaining components standalone, i.e., outside
of the context of any particular platform. This is primarily intended
for build time testing.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>