These days arm64 kernels are always SMP, and thus smp_dmb is an
overly-long way of writing dmb. Naturally, no-one uses it.

Remove the unused macro.
Signed-off-by: NMark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: NWill Deacon <will@kernel.org>