arm: Implement l2x0 cache disable functions

Add flush_all, inv_all and disable functions to the l2x0 code. These
functions are called from kexec code to prevent random crashes in the
new kernel.

Platforms like OMAP which control L2 enable/disable via SMI mode can
override the outer_cache.disable() function to implement their own.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Linus Walleij <linus.walleij@stericsson.com>