SecurityPkg/RngDxe: Conditionally install EFI_RNG_PROTOCOL

On Arm platforms, the number of available RNG algorithms is
dynamically detected and can be 0 in the absence of FEAT_RNG
and firmware TRNG.
In this case, the EFI_RNG_PROTOCOL should not be installed to
prevent from installing an empty protocol.

Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
[ardb: return EFI_REQUEST_UNLOAD_IMAGE instead of an error]
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>