ARM: dts: vf610-twr: Add ADC support

vf610 has two ADC controllers, and vf610-twr board ADC0_SE5 pin connect
to sliding rheostat for ADC test, other ADC pins connect to connectors for
future use.

Add support for ADC0_SE5.

CC: Jonathan Cameron <jic23@kernel.org>
CC: Mark Rutland <mark.rutland@arm.com>
CC: Otavio Salvador <otavio@ossystems.com.br>
CC: Peter Meerwald <pmeerw@pmeerw.net>
CC: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>