lan78xx: Read LED states from Device Tree

Add support for DT property "microchip,led-modes", a vector of two
cells (u32s) in the range 0-15, each of which sets the mode for one
of the two LEDs. The possible values are:

    0=link/activity          1=link1000/activity
    2=link100/activity       3=link10/activity
    4=link100/1000/activity  5=link10/1000/activity
    6=link10/100/activity    14=off    15=on

Signed-off-by: Phil Elwell <phil@raspberrypi.org>