MdeModulePkg Variable: Enhance variable performance by reading from existed memory...

MdeModulePkg Variable: Enhance variable performance by reading from existed memory cache.

Current variable driver already have memory cache to improve performance.
Change the code which read from physical MMIO address to read from memory cache.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Derek Lin <derek.lin2@hpe.com>
Reviewed-by: Samer El-Haj-Mahmoud <elhaj@hpe.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18654 6f19259b-4bc3-4df7-8a09-765794883524