ARM: 5994/1: ARM: Add outer_cache_fns.sync function pointer (2/4)

This patch introduces the outer_cache_fns.sync function pointer together
with the OUTER_CACHE_SYNC config option that can be used to drain the
write buffer of the outer cache.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>