iio: adc: add ADC12130/ADC12132/ADC12138 ADC driver

This adds Texas Instruments' ADC12130/ADC12132/ADC12138 12-bit plus
sign ADC driver.  I have tested with the ADC12138.  The ADC12130 and
ADC12132 are not tested but these are similar to ADC12138 except that
the mode programming instruction is a bit different.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>