hwmon: add Maxim MAX197 support

The MAX197 is an A/D converter, made by Maxim. This driver currently
supports the MAX197, and MAX199. They are both 8-Channel, Multi-Range,
5V, 12-Bit DAS with 8+4 Bus Interface and Fault Protection.

The available ranges for the MAX197 are {0,-5V} to 5V, and {0,-10V} to
10V, while they are {0,-2V} to 2V, and {0,-4V} to 4V on the MAX199.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/max197

@@ -0,0 +1,60 @@
+Maxim MAX197 driver
+===================
+
+Author:
+  * Vivien Didelot <vivien.didelot@savoirfairelinux.com>
+
+Supported chips:
+  * Maxim MAX197
+    Prefix: 'max197'
+    Datasheet: http://datasheets.maxim-ic.com/en/ds/MAX197.pdf
+
+  * Maxim MAX199
+    Prefix: 'max199'
+    Datasheet: http://datasheets.maxim-ic.com/en/ds/MAX199.pdf
+
+Description
+-----------
+
+The A/D converters MAX197, and MAX199 are both 8-Channel, Multi-Range, 5V,
+12-Bit DAS with 8+4 Bus Interface and Fault Protection.
+
+The available ranges for the MAX197 are {0,-5V} to 5V, and {0,-10V} to 10V,
+while they are {0,-2V} to 2V, and {0,-4V} to 4V on the MAX199.
+
+Platform data
+-------------
+
+The MAX197 platform data (defined in linux/platform_data/max197.h) should be
+filled with a pointer to a conversion function, defined like:
+
+    int convert(u8 ctrl);
+
+ctrl is the control byte to write to start a new conversion.
+On success, the function must return the 12-bit raw value read from the chip,
+or a negative error code otherwise.
+
+Control byte format:
+
+Bit     Name       Description
+7,6     PD1,PD0    Clock and Power-Down modes
+5       ACQMOD     Internal or External Controlled Acquisition
+4       RNG        Full-scale voltage magnitude at the input
+3       BIP        Unipolar or Bipolar conversion mode
+2,1,0   A2,A1,A0   Channel
+
+Sysfs interface
+---------------
+
+* in[0-7]_input: The conversion value for the corresponding channel.
+                 RO
+
+* in[0-7]_min:   The lower limit (in mV) for the corresponding channel.
+                 For the MAX197, it will be adjusted to -10000, -5000, or 0.
+                 For the MAX199, it will be adjusted to -4000, -2000, or 0.
+                 RW
+
+* in[0-7]_max:   The higher limit (in mV) for the corresponding channel.
+                 For the MAX197, it will be adjusted to 0, 5000, or 10000.
+                 For the MAX199, it will be adjusted to 0, 2000, or 4000.
+                 RW