MdeModulePkg/Bds: Refine the code to load file from FV.

MdeModulePkg/Bds: Refine the code to load file from FV.

Change BmGetFileBufferByMemmapFv to BmGetFileBufferByFvFilePath.
The original function gets the file buffer only from memory mapped
FV device path and leaves GUIDed FV device path to the code below;
The new function gets the file buffer from both formats of FV device
paths.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Sunny Wang <sunnywang@hpe.com>