ad525x_dpot: new driver for AD525x digital potentiometers

This driver supports the non-volatile digital potentiometers via I2C:
AD5258, AD5259, AD5251, AD5252, AD5253, AD5254, and AD5255

It provides a sysfs interface to each device for reading/writing which
is documented in Documentation/misc-devices/ad525x_dpot.txt.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Chris Verges <chrisv@cyberswitching.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Cc: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/misc-devices/ad525x_dpot.txt

@@ -0,0 +1,57 @@
+---------------------------------
+  AD525x Digital Potentiometers
+---------------------------------
+
+The ad525x_dpot driver exports a simple sysfs interface.  This allows you to
+work with the immediate resistance settings as well as update the saved startup
+settings.  Access to the factory programmed tolerance is also provided, but
+interpretation of this settings is required by the end application according to
+the specific part in use.
+
+---------
+  Files
+---------
+
+Each dpot device will have a set of eeprom, rdac, and tolerance files.  How
+many depends on the actual part you have, as will the range of allowed values.
+
+The eeprom files are used to program the startup value of the device.
+
+The rdac files are used to program the immediate value of the device.
+
+The tolerance files are the read-only factory programmed tolerance settings
+and may vary greatly on a part-by-part basis.  For exact interpretation of
+this field, please consult the datasheet for your part.  This is presented
+as a hex file for easier parsing.
+
+-----------
+  Example
+-----------
+
+Locate the device in your sysfs tree.  This is probably easiest by going into
+the common i2c directory and locating the device by the i2c slave address.
+
+	# ls /sys/bus/i2c/devices/
+	0-0022  0-0027  0-002f
+
+So assuming the device in question is on the first i2c bus and has the slave
+address of 0x2f, we descend (unrelated sysfs entries have been trimmed).
+
+	# ls /sys/bus/i2c/devices/0-002f/
+	eeprom0 rdac0 tolerance0
+
+You can use simple reads/writes to access these files:
+
+	# cd /sys/bus/i2c/devices/0-002f/
+
+	# cat eeprom0
+	0
+	# echo 10 > eeprom0
+	# cat eeprom0
+	10
+
+	# cat rdac0
+	5
+	# echo 3 > rdac0
+	# cat rdac0
+	3