ARM i.MX: Update defconfig

This updates the defconfig for i.MX3, i.MX5 and i.MX6.

- all these can be compiled together now, so remove mx3_defconfig and
  mx5_defconfig and introduce a imx_v6_v7_defconfig instead.

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