dts: bcm2711: Add 'eee' dtparam

The 'eee' dtparam allows EEE to be disabled on the onboard Ethernet
controller. Adding 'dtparam=eee=off' to config.txt causes
'genet.eee=N' to be added to the kernel command line, which in turn
tells the Genet driver to disable EEE.

See: https://github.com/raspberrypi/linux/issues/4289



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