ARM: 7957/1: add DSB after icache flush in __flush_icache_all()

Add DSB after icache flush to complete the cache maintenance operation.

Signed-off-by: Vinayak Kale <vkale@apm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>