MdeModulePkg/Capsule: Remove RT restriction in UpdateCapsule service.

MdeModulePkg/Capsule: Remove RT restriction in UpdateCapsule service.

Current UpdateCapsule service will reject all non-reset capsule images and
return EFI_OUT_OF_RESOURCE if the system is at runtime. This will block a
platform CapsuleLib from implementing ProcessCapsuleImage() with runtime
capsule processing capability.

This patch removes this restriction. The change is controled by a feature
PCD PcdSupportProcessCapsuleAtRuntime, and the default value is FALSE
which means not enable this feature.

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2501

Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Signed-off-by: Siyuan Fu <siyuan.fu@intel.com>