Platform/Sgi: Cleanup build options for StandaloneMM context

The Arm reference design platforms support only AArch64 mode for
StandaloneMM execution context. So cleanup the existing build options
specified for StandaloneMM.

Signed-off-by: Omkar Anand Kulkarni <omkar.kulkarni@arm.com>
Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>
Reviewed-by: Thomas Abraham <thomas.abraham@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>