ARM: mach-ux500: unlock I&D l2x0 caches before init

Apparently U8500 U-Boot versions may leave the l2x0 locked down
before executing the kernel. Make sure we unlock it before we
initialize the l2x0. This fixes a performance problem reported
by Jan Rinze.

The l2x0 core has been modified to unlock the l2x0 by default,
but it will not touch the locking registers if the l2x0 was
already enabled, as on the ux500, so we need this quirk to
make sure it is properly turned off.

Cc: stable@kernel.org
Cc: Srinidhi Kasagar <srinidhi.kasagar@stericsson.com>
Cc: Rabin Vincent <rabin.vincent@stericsson.com>
Cc: Adrian Bunk <adrian.bunk@movial.com>
Reported-by: Jan Rinze <janrinze@gmail.com>
Tested-by: Robert Marklund <robert.marklund@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>