SecurityPkg/RngDxe: Replace Pcd with Sp80090Ctr256Guid

gEfiRngAlgorithmSp80090Ctr256Guid was used as the default algorithm
in RngGetRNG(). The commit below set the default algorithm to
PcdCpuRngSupportedAlgorithm, which is a zero GUID by default.

As the Pcd value is not defined for any platform in the edk2-platfoms
repository, assume it was an error and go back to the first version,
using gEfiRngAlgorithmSp80090Ctr256Guid.

Fixes: 4e5ecdba ("SecurityPkg: Add support for RngDxe on AARCH64")
Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Acked-by: Jiewen Yao <jiewen.yao@intel.com>