ArmPkg: implement ResetSystemLib using PSCI 0.2 calls

This adds an implementation of the ResetSystemLib library class as
defined in MdeModulePkg. It is used as the platform glue by the generic
ResetSystemRuntimeDxe which lives in the same package.

This implementation is intended to replace the EfiResetSystemLib based
implementation that is deprecated now that we have decided that there is
no longer a reason to keep a different ResetSystem() implementation
under EmbeddedPkg.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>