FatPkg: Break down Part.c file.

FatPkg: Break down Part.c file.

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=1470
Break down partition parsing logic to 2 parts, Eltorito and MBR.

Cc: Ruiyu Ni <ray.ni@intel.com>
Cc: Zhang Chao B <chao.b.zhang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Chen A Chen <chen.a.chen@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>