ArmVirtPkg: Add Null instance of ArmCcaLib

Add a Null instance of ArmCcaLib so that guest firmware that does
not support Arm CCA can link to this Null version of the library.

Also include it in ArmVirt.dsc.inc so that it is linked for the
non-Arm CCA firmware builds.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>