SecurityPkg: Add support for RngDxe on AARCH64

AARCH64 support has been added to BaseRngLib via the optional
ARMv8.5 FEAT_RNG.

Refactor RngDxe to support AARCH64, note support for it in the
VALID_ARCHITECTURES line of RngDxe.inf and enable it in SecurityPkg.dsc.

Signed-off-by: Rebecca Cran <rebecca@nuviainc.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Acked-by: Jiewen Yao <Jiewen.yao@intel.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>