MdeModulePkg DxeCore: Handle FFS file with FFS_ATTRIB_CHECKSUM set for not cache...

MdeModulePkg DxeCore: Handle FFS file with FFS_ATTRIB_CHECKSUM set for not cache memory mapped IO FV.

The code FvCheck() will check FFS file checksum to verify if FFS file is valid when the file is
with FFS_ATTRIB_CHECKSUM set. The whole file will be read through when doing checksum check.
So we can cache FFS file to memory buffer for following checksum calculating first.
And then, the cached file buffer can be also used for FvReadFile.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15958 6f19259b-4bc3-4df7-8a09-765794883524