ArmPkg: MTL Library interface and Null library implementation
Upcoming new component ArmPkg/Drivers/ArmScmiDxe is dependent on
platform specific ArmMtlLib library implementation, however in order
to be able to build the ArmScmiDxe component outside of the context of a
particular platform, this change adds Null implementation of the
ArmMtlLib along with ARM MTL library header.