net: phy: broadcom: Add support for setting LED brightness

Broadcom PHYs have two LEDs selector registers which allow us to control
the LED assignment, including how to turn them on/off.

Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>