MdeModulePkg PiSmmCore: Retrieve Smram base address from system table

MdeModulePkg PiSmmCore: Retrieve Smram base address from system table

PiSmmIpl records LoadModuleAtFixAddressSmramBase in LMFAConfigurationTable.
Update PiSmmCore to directly get the address from this system table.

Cc: Star Zeng <star.zeng@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>