MdeModulePkg/PiSmmCore: switch to MdePkg allocation granularity macros

Remove the local definitions for the default and runtime page allocation
granularity macros, and switch to the new MdePkg versions.

Note that this replaces a reference to the 'default' version with the
more correct 'runtime' version, but this matters little in practice.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>