ArmPlatformPkg: Use EfiEventGroupSignal from UefiLib

ArmPlatformPkg: Use EfiEventGroupSignal from UefiLib

Use EfiEventGroupSignal from UefiLib and remove
EmptyCallbackFunction.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=298

Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>