drm/radeon: add hwmon interface for managing fan pwm (v2)

This adds percent-based fan control. Attributes (I hope) follow the
sysfs-interface specification:
* pwm1 - fan speed query/manage
* pwm1_max, pwm1_min - min/max values for fan pwm (constant)
* pwm1_enable - fan control query/manage (enable/disable)

(There is no rpm-based control for now)

v2: agd5f: fix formatting

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