StandaloneMmPkg: Zero data structure explicitly

StandaloneMmPkg: Zero data structure explicitly

Introduction of the -mstrict-align flag results in GCC attempting
to use memset to zero out the InitMmFoundationSvcArgs structure.
In the absence of this C library function, this patch explicitly
zeroes this data structure prior to use.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Achin Gupta <achin.gupta@arm.com>
Signed-off-by: Sughosh Ganu <sughosh.ganu@arm.com>
Reviewed-by: Achin Gupta <achin.gupta@arm.com>