docs: hwmon: Document max127 driver

Add documentation for the max127 hardware monitoring driver.
Signed-off-by: Tao Ren <rentao.bupt@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20201123185658.7632-3-rentao.bupt@gmail.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/index.rst

@@ -111,6 +111,7 @@ Hardware Monitoring Kernel Drivers
    ltc4245
    ltc4260
    ltc4261
+   max127
    max16064
    max16065
    max1619

Documentation/hwmon/max127.rst

+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+Kernel driver max127
+====================
+
+Author:
+
+  * Tao Ren <rentao.bupt@gmail.com>
+
+Supported chips:
+
+  * Maxim MAX127
+
+    Prefix: 'max127'
+
+    Datasheet: https://datasheets.maximintegrated.com/en/ds/MAX127-MAX128.pdf
+
+Description
+-----------
+
+The MAX127 is a multirange, 12-bit data acquisition system (DAS) providing
+8 analog input channels that are independently software programmable for
+a variety of ranges. The available ranges are {0,5V}, {0,10V}, {-5,5V}
+and {-10,10V}.
+
+The MAX127 features a 2-wire, I2C-compatible serial interface that allows
+communication among multiple devices using SDA and SCL lines.
+
+Sysfs interface
+---------------
+
+  ============== ==============================================================
+  in[0-7]_input  The input voltage (in mV) of the corresponding channel.
+		 RO
+
+  in[0-7]_min    The lower input limit (in mV) for the corresponding channel.
+		 ADC range and LSB will be updated when the limit is changed.
+		 For the MAX127, it will be adjusted to -10000, -5000, or 0.
+		 RW
+
+  in[0-7]_max    The higher input limit (in mV) for the corresponding channel.
+		 ADC range and LSB will be updated when the limit is changed.
+		 For the MAX127, it will be adjusted to 0, 5000, or 10000.
+		 RW
+  ============== ==============================================================