ARM i.MX5: update defconfig

Rename it from mx51_defconfig to mx5_defconfig since the i.MX53
is now supported aswell. Also, remove the DEBUG_LL and enable
some more drivers useful for i.MX5 boards. For supporting
1GiB of SDRAM vmsplit 2/2 is enabled.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>