hwmon: add fan speed monitoring driver for Surface devices

hwmon: add fan speed monitoring driver for Surface devices

Adds a driver that provides read only access to the fan speed for Microsoft
Surface Pro devices. The fan speed is always regulated by the EC and cannot
be influenced directly.

Signed-off-by: Ivor Wanders <ivor@iwanders.net>
Link: https://github.com/linux-surface/kernel/pull/144
Reviewed-by: Maximilian Luz <luzmaximilian@gmail.com>
Reviewed-by: Armin Wolf <W_Armin@gmx.de>
Link: https://lore.kernel.org/r/20240131005856.10180-2-ivor@iwanders.net
[groeck:
 - Declare surface_fan_hwmon_is_visible() static
 - Add dependency on SURFACE_AGGREGATOR_BUS
]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>