StandaloneMmPkg: Fix ECC error 4002 and 9002 in StandaloneMmCore

StandaloneMmPkg: Fix ECC error 4002 and 9002 in StandaloneMmCore

Bugzilla: 3150 (https://bugzilla.tianocore.org/show_bug.cgi?id=3150)

Add doxygen style function documentation headers to fix the ECC
reported errors:
 - [4002] Function header doesn't exist.
 - [9002] The function headers should follow Doxygen special
   documentation blocks in section 2.3.5.

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Acked-by: Jiewen Yao <Jiewen.yao@intel.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>