l2c_enable() is documented that it must not be called if the cache has
already been enabled.  Unfortunately, commit 6b49241a ("ARM: 8259/1:
l2c: Refactor the driver to use commit-like interface") changed this
without updating the comment, for very little reason.  Revert this
change and restore the expected behaviour.
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>