MdePkg/ArmTrngLib: Add NULL instance of Arm TRNG Library

MdePkg/ArmTrngLib: Add NULL instance of Arm TRNG Library

Bugzilla: 3668 (https://bugzilla.tianocore.org/show_bug.cgi?id=3668)

The Arm True Random Number Generator (TRNG) library defines an
interface to access the entropy source on a platform. On platforms
that do not have access to an entropy source, a NULL instance of
the TRNG library may be useful to satisfy the build dependency.

Therefore, add a NULL instance of the Arm TRNG library.

Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>