iio: dac: mcp4725: Add basic support for MCP4726

iio: dac: mcp4725: Add basic support for MCP4726

MCP4726 is a single channel 12-bit DAC.  We can support MCP4726 with
a little changes to mcp4725 driver.  In power-down mode, they have
different selection of VOUT pull-down registers.

MCP4726 also has features:
 - Output gain options: 1x, 2x
 - Voltage reference selection: VDD, VREF (Unbuffered or Buffered)

But these are not supported in this change. (1x gain, VDD is selected)

datasheet: http://ww1.microchip.com/downloads/en/DeviceDoc/22272C.pdf

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>
Cc: linux-iio@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>