StandaloneMmPkg: StandaloneMmCoreMemoryAllocationLib: Fix compiler warning

StandaloneMmPkg: StandaloneMmCoreMemoryAllocationLib: Fix compiler warning

Assigning MmramRangeCount from MmCorePrivate (UINT64) to local variable
MmramRangeCount (UINT32) will cause compilation failure due to "warning
C4244: '=': conversion from 'UINT64' to 'UINT32', possible loss of data".
This changes defines local MmramRangeCount as UINTN type and adds type
cast before value assignment.

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Sami Mujawar <sami.mujawar@arm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Supreeth Venkatesh <supreeth.venkatesh@arm.com>
Signed-off-by: Kun Qin <kun.q@outlook.com>
Reviewed-by: Jiewen Yao <Jiewen.yao@intel.com>