dt-bindings: hwmon: Increase max number of io-channels

The Analog Monitoring System (AMS) defines 51 channels
(Documentation/devicetree/bindings/iio/adc/xlnx,zynqmp-ams.yaml)
that's why increase number to 51.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/5110a313a5ce52ce6d7b5cb6b08368d42063dc30.1701419691.git.michal.simek@amd.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>