arm/imx: change mxc_init_l2x0() to an imx31/35 specific call

The mxc_init_l2x0() should really be an imx31/35 specific call.
The patch removes early_initcall from mxc_init_l2x0() and get imx31
and imx35 soc specific function calls mxc_init_l2x0(), so that it's
not necessarily to be called for all imx socs when we build single
image for multiple imx socs.

Thus the function can be renamed to imx3_init_l2x0() and put into
mm-imx3.c.  It also changes the return type from integer to void.
From what I see, the integer was picked just to satisfy early_initcall
prototype.

With the patch 'ARM: l2x0: add empty l2x0_of_init' applied, the code
compiles even without CONFIG_CACHE_L2X0 enabled.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>