As PHY address 0 is not used on PHYTEC i.MX 6 and i.MX 6UL SOMs we do
not have to override the B-CAST_OFF strap-in which disables broadcast
on PHY address 0.

Also add some comments about the magic values.

Signed-off-by: Jan Remmet <j.remmet@phytec.de>
Signed-off-by: Stefan Riedmueller <s.riedmueller@phytec.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>