ARM: l2c: only unlock caches if NS_LOCKDOWN bit is set

Some L2C caches have a bit which allows non-secure software to control
the cache lockdown.  Some platforms are unable to set this bit.  To
avoid receiving an abort while trying to unlock the cache lines, check
the state of this bit before unlocking.  We do this by providing a new
method in the l2c_init_data to perform the unlocking.

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