iio: adc: add support for ADC0831/ADC0832/ADC0834/ADC0838 chips

This adds ADC0831/ADC0832/ADC0834/ADC0838 8-bit ADC driver.
I have tested with ADC0831 and ADC0832.  The remaining ADC0834 and
ADC0838 are very similar to ADC0832.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
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>