add missing i2c4 clock and correct the compatible string to match other
imx6 i2c blocks

Signed-off-by: Iain Paton<ipaton0@gmail.com>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>