arm64: barriers: add dmb barrier

Commit 8adbf57fc429 ("irqchip: gic: use dmb ishst instead of dsb when
raising a softirq") added an explicit dmb(...) call to the GIC driver.

This patch adds a simple dmb() macro to arm64, which expands to a DMB SY
instruction.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Git-commit: d152d22a18c240286c19997a6249ee76ea055926
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>