iio: mcp4531: provide range of available raw values

Example:

$ cat '/sys/bus/iio/devices/iio:device0/out_resistance_raw_available'
[0 1 256]

Meaning: min 0, step 1 and max 256.

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index ec1ee3e..1395246 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -7709,6 +7709,7 @@
 M:	Peter Rosin <peda@axentia.se>
 L:	linux-iio@vger.kernel.org
 S:	Maintained
+F:	Documentation/ABI/testing/sysfs-bus-iio-potentiometer-mcp4531
 F:	drivers/iio/potentiometer/mcp4531.c
 
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