BCM270X_DT: Add new Ethernet DT parameters

BCM270X_DT: Add new Ethernet DT parameters

Add "eth_downshift_after" DT parameter to allow the delay before the
downshift to be specified. The default is 2 auto-negotiation cycles,
and legal values are 2, 3, 4, 5 and 0 (disabled).

Add "eth_max_speed" DT parameter as a way of prohibiting 1000Mbps
links. This can be used to avoid the delay until the downshift mechanism
activates. Legal values are 10, 100 and 1000, where the default is
unlimited (effectively 1000Mbps).

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