BCM27XX_DT: Set LED modes from Device Tree

The new default values for LAN7515 (LAN7800) are:

  LED0 = 1 (link1000/activity)
  LED1 = 6 (link10/link100/activity)

Also add two dtparams - eth_led0 and eth_led1 - to provide user control
over the LEDs.

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