coreboot: Add manual fan control

In automatic mode, EC is responsible for controlling fan duty as part of
thermal management. This is the existing behavior.

In manual mode, EC does not control fan duty target. The OS is
responsible for controlling duty for thermal management.

Control by system firmware is not yet implemented.

This allows users to control fan duty via ACPI.

RPM target is not supported.

Signed-off-by: Tim Crawford <tcrawford@system76.com>