MdeModulePkg/PiSmmCore: use EfiPagesToSize to prevent build error.

MdeModulePkg/PiSmmCore: use EfiPagesToSize to prevent build error.

EFI_PAGES_TO_SIZE only handles UINTN, so we use EfiPagesToSize
to handle UINT64.

Cc: Jeff Fan <jeff.fan@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>