Add imx6ul touchscreen controller support.

TSC module need ADC2 module to measure the touchscreen
coordinate value. This patch put TSC and ADC2 together,
make ADC2 module only be used for TSC, can't be used as
a normal ADC.
Signed-off-by: Haibo Chen <haibo.chen@freescale.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>