arm64: barriers: use barrier() instead of smp_mb() when !SMP

arm64: barriers: use barrier() instead of smp_mb() when !SMP

The recently introduced acquire/release accessors refer to smp_mb()
in the !CONFIG_SMP case. This is confusing when reading the code, so use
barrier() directly when we know we're UP.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
(cherry picked from commit be6209a6107e0f63544e3e7d00fd5c95434ec80a)
Signed-off-by: Mark Brown <broonie@kernel.org>
Conflicts:
	arch/arm64/include/asm/barrier.h