dt-bindings: iio: ltc2632: expand for ltc2636 support

The ltc2636 family of devices is register compatible with the ltc2636
chips, it just features 8 instead of 2 channels.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/dac/ltc2632.txt

-Linear Technology LTC2632 DAC device driver
+Linear Technology LTC2632/2636 DAC
 
 Required properties:
  - compatible: Has to contain one of the following:
@@ -8,6 +8,12 @@ Required properties:
 	lltc,ltc2632-h12
 	lltc,ltc2632-h10
 	lltc,ltc2632-h8
+	lltc,ltc2636-l12
+	lltc,ltc2636-l10
+	lltc,ltc2636-l8
+	lltc,ltc2636-h12
+	lltc,ltc2636-h10
+	lltc,ltc2636-h8
 
 Property rules described in Documentation/devicetree/bindings/spi/spi-bus.txt
 apply. In particular, "reg" and "spi-max-frequency" properties must be given.