r8169: add support for LED's on RTL8168/RTL8101

This adds support for the LED's on most chip versions. Excluded are
the old non-PCIe versions and RTL8125. RTL8125 has a different LED
register layout, support for it will follow later.

LED's can be controlled from userspace using the netdev LED trigger.

Tested on RTL8168h.

Note: The driver can't know which LED's are actually physically
wired. Therefore not every LED device may represent a physically
available LED.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>