SecurityPkg Variable: Support the new introduced PcdMaxAuthVariableSize.

SecurityPkg Variable: Support the new introduced PcdMaxAuthVariableSize.

1. If PcdMaxAuthVariableSize is set to 0, keep current behavior as is and
PcdMaxVariableSize used.
2. If PcdMaxAuthVariableSize is set to non 0, it will work on
authenticated variables.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17610 6f19259b-4bc3-4df7-8a09-765794883524