iio: adc: mcp3422: Add support for MCP3425

The MCP3425 is a single channel up to 16-bit A/D converter which has
features:

- On-Board Programmable Gain Amplifier (PGA):
 - Gains of 1, 2, 4 or 8
- Programmable Data Rate Options:
 - 15 SPS (16 bits), 60 SPS (14 bits), 240 SPS (12 bits)

The mcp3422 driver also supports the MCP3421 which is a single channel.
So we can support MCP3425 with a little changes to mcp3422 driver.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Cc: Peter Meerwald <pmeerw@pmeerw.net>
Cc: linux-iio@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>