ARM: l2c: ux500: implement dummy write_sec method

ux500 can't write to any of the secure registers on the L2C controllers,
so provide a dummy handler which ignores all writes.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>