irqchip/gic-v3-its: Specialise flush_dcache operation

It'd be better to switch to CMA... but before that done redirect
flush_dcache operation, so 32-bit implementation could be wired
latter.

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>