StandaloneMmPkg: Fix ECC error 4002 in StandaloneMmCoreEntryPoint

StandaloneMmPkg: Fix ECC error 4002 in StandaloneMmCoreEntryPoint

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

Add doxygen style function headers to fix ECC reported errors
"[4002] Function header doesn't exist Function".

Also update the corresponding declaration to add IN/OUT doxygen
style reference tags.

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>