Add L2 cache support and enable it by default.

Configure the L2 cache in the same way as done by FSL kernel:
http://git.freescale.com/git/cgit.cgi/imx/linux-2.6-imx.git/tree/arch/arm/mach-mx6/mm.c?h=imx_3.0.35_4.1.0



Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Dirk Behme <dirk.behme@gmail.com>
Acked-by: Stefano Babic <sbabic@denx.de>