BeagleBoardPkg: switch to use MdeModulePkg ResetSystemLib

BeagleBoardPkg: switch to use MdeModulePkg ResetSystemLib

The BeagleBoard port used EmbeddedPkg/ResetRuntimeDxe/ResetRuntimeDxe.inf
for its reset handling. With the arrival
MdeModulePkg/Universal/ResetSystemRuntimeDxe/ResetSystemRuntimeDxe.inf

As part of this, change BeagleBoardPkg/Library/ResetSystemLib to be an
implementation of ResetSystemLib instead of the previous
EfiResetSystemLib.

Wire all reset variants to ResetCold, except for ResetShutdown and
EnterS3WithImmediateWake, which return immediately.

Note: this ResetSystemLib never supported being called after
ExitBootservices, and this shortcoming is not addressed here.

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