MdeModulePkg: VariableSmmRuntimeDxe: Added request unblock memory interface

MdeModulePkg: VariableSmmRuntimeDxe: Added request unblock memory interface

This changes added usage of MmUnblockMemoryLib to explicitly request
runtime cache regions(and its indicators) to be accessible from MM
environment when PcdEnableVariableRuntimeCache is enabled. It will bring
in compatibility with architectures that supports full memory blockage
inside MM.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Signed-off-by: Kun Qin <kun.q@outlook.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>
Message-Id: <MWHPR06MB31026EA41F28F2CE12B68574F3969@MWHPR06MB3102.namprd06.prod.outlook.com>