hwmon: (amc6821) Move reading fan data from OF to a function

Move fan property reading from OF to a separate function. This keeps OF
data handling separate from the code logic and makes it easier to add
features like cooling device support that use the same fan node.

Signed-off-by: João Paulo Gonçalves <joao.goncalves@toradex.com>
Link: https://lore.kernel.org/r/20250613-b4-amc6821-cooling-device-support-v4-2-a8fc063c55de@toradex.com


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