iio: adc: vf610: Add IIO buffer support for Vybrid ADC

This patch adds support for IIO buffer to the Vybrid ADC driver.
IIO triggered buffer infrastructure along with iio sysfs trigger
is used to leverage continuous sampling support provided by the
ADC block.

Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>
Acked-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>