ARM: l2c: omap2: implement new write_sec method

With the write_sec method, we no longer need to override the default
L2C disable method, and we no longer need the L2C set_debug method.
Both of these can be handled via the write_sec method.

Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>