asus-wmi: Add support for custom fan curves

Add support for custom fan curves found on some ASUS ROG laptops.

These laptops have the ability to set a custom curve for the CPU
and GPU fans via two ACPI methods.

This patch adds two pwm<N> attributes to the hwmon sysfs,
pwm1 for CPU fan, pwm2 for GPU fan. Both are under the hwmon of the
name `asus_custom_fan_curve`. There is no safety check of the set
fan curves - this must be done in userspace.

The fans have settings [1,2,3] under pwm<N>_enable:
1. Enable and write settings out
2. Disable and use factory fan mode
3. Same as 2, additionally restoring default factory curve.

Use of 2 means that the curve the user has set is still stored and
won't be erased, but the laptop will be using its default auto-fan
mode. Re-enabling the manual mode then activates the curves again.

Notes:
- pwm<N>_enable = 0 is an invalid setting.
- pwm is actually a percentage and is scaled on writing to device.

Signed-off-by: Luke D. Jones <luke@ljones.dev>