add common fan control asic callbacks

Adds 4 callbacks for managing fan state/speed
* fan_ctrl_set_mode - manages fan control mode (nonzero == manual)
* fan_ctrl_get_mode - queries fan control mode
* set_fan_speed_percent - manages fan speed as percentage value
* get_fan_speed_percent - queries fan speed as percentage value

Signed-off-by: Oleg Chernovskiy <algonkvel@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>