MdeModulePkg/PiSmmCore: remove NX attr for SMM RAM

MdeModulePkg/PiSmmCore: remove NX attr for SMM RAM

If PcdDxeNxMemoryProtectionPolicy is set to enable protection for memory
of EfiReservedMemoryType, the BIOS will hang at a page fault exception
during starting SMM driver.

The root cause is that SMM RAM is type of EfiReservedMemoryType and
marked as non-executable. The fix is simply removing NX attribute for
those memory.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>