arm64: barriers: add dmb barrier

Commit 8adbf57f

 ("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>