Make laptop fan duty pwm writable

[curiousercreative]

All sysfs paths assume that /sys/class/hwmon/hwmon3/name === "system76_acpi", so check which /sys/class/hwmon/hwmonX is the one for your system.

1. makes /sys/class/hwmon/hwmon3/pwm1 and /sys/class/hwmon/hwmon3/pwm2 (where available) writable. Write 0-255 to set a desired fan speed where 255 = 100%.
2. When writes are received to sysfs, we forward to ACPI methods SFD0 and SFD1 which should get forwarded all the way to the EC fans controller. (SFD = Set Fan Duty)

Further discussion/consideration needed for:

• how to range clamp the values written to sysfs? or should we return an error?
• should we also make pwn_enable writable and pass that all the way through to the EC?

Relates to (requires for new functionality, shouldn't break though):

• Allow ACPI to set fan speeds system76/ec#309
• Add ACPI methods for setting fan speed system76/coreboot#143

Along with related PRs, would resolve/improve:

• [Feature Request] Un-hardcoded fan curves definable within OS system76/firmware-open#570
• [Feature Request] Allow setting fan speed by writing to sysfs #9

[curiousercreative]

• rebased, tested again on my galp5.
• simplified this PR to just the fans being writable, README suggestions in Add build and install instructions #26