ArmPlatformPkg: convert VExpress ResetSystemLib to ResetSystemLib

Since we're in the process of migrating all of the VExpress platforms
to MdeModulePkg ResetSystemRuntimeDxe, convert VExpress ResetSystemLib
from EfiResetSystemLib interface to the ResetSystemLib one.

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