ARM: i.MX: initialize l2x0 at early_init time

This can make the l2 cache work at early time which
will benefit the boot up time.

Tested on i.mx31pdk board, test result shows:

Before: arch_initcall(mxc_init_l2x0);

[    0.558755] console [ttymxc0] enabled

After: early_initcall(mxc_init_l2x0);

[    0.555716] console [ttymxc0] enabled

Signed-off-by: Jason Liu <jason.hui@linaro.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>