Add the TPL raise/restore code for VariableSmmRuntimeDxe to avoid variable services reentry.

Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14038 6f19259b-4bc3-4df7-8a09-765794883524