ARM: l2x0: add empty l2x0_of_init

Add empty version of l2x0_of_init for when CONFIG_CACHE_L2X0 is not selected.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Barry Song <21cnbao@gmail.com>
Reviewed-by: Shawn Guo <shawn.guo@linaro.org>