iio:adc:imx: add Freescale Vybrid vf610 adc driver

Add Freescale Vybrid vf610 adc driver. The driver only support
ADC software trigger.

VF610 ADC device documentation is available at below reference manual
(chapter 37):
http://cache.freescale.com/files/32bit/doc/ref_manual/VYBRIDRM.pdf?
fpsp=1&WT_TYPE=Reference%20Manuals&WT_VENDOR=FREESCALE&WT_FILE_FORMAT
=pdf&WT_ASSET=Documentation

CC: Shawn Guo <shawn.guo@linaro.org>
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: Jonathan Cameron <jic23@kernel.org>