StandaloneMmPkg: Describe the declaration and definition files.

StandaloneMmPkg: Describe the declaration and definition files.

This patch describes the package declarations and definitions
for creating standalone management mode image with
core/foundation and drivers.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Supreeth Venkatesh <supreeth.venkatesh@arm.com>
Reviewed-by: Achin Gupta <achin.gupta@arm.com>
Reviewed-by: Jiewen Yao <Jiewen.yao@intel.com>
Signed-off-by: Sughosh Ganu <sughosh.ganu@arm.com>