net: phy: dp83867: Add led_brightness_set support

Up to 4 LEDs can be attached to the PHY, add support for setting
brightness manually.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20230424134625.303957-1-alexander.stein@ew.tq-group.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>