ArmPkg/ArmSvcLib: Return x4-x7 in output parameters

ArmPkg/ArmSvcLib: Return x4-x7 in output parameters

The Arm SMC calling convention standard v1.2 allows 8 input and output
parameter registers. The FF-A specification relies on this
communication. This patch extends the number of output registers
returned by ArmCallSvc() to match this convention.

Signed-off-by: Achin Gupta <achin.gupta@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>