ARM: imx: let L2 initialization be a common function

Move imx6q L2 initialization function imx6q_init_l2cache() into
system.c, and rename it imx_init_l2cache(), so that other platforms
other than imx6q can also use the function.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Dirk Behme <dirk.behme@de.bosch.com>