StandaloneMmPkg: switch to MM communicate 2 protocol

StandaloneMmPkg: switch to MM communicate 2 protocol

Update the reference to MM communicate to refer to the MM communicate 2
protocol instead. This makes no difference for the MM side of the
implementation, but is more accurate nonetheless, since the original MM
protocol does not work in combination with standalone MM.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>