ARM: l2c: implement L2C-310 erratum 727915 as a method override

Implement L2C-310 erratum 727915 by overriding the flush_all method
in the outer_cache operations structure.  This allows us to sensibly
contain the erratum code in one place without affecting other
locations or implementations.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>