arm: bcmcygnus: Enable Ethernet support

Enable BCM SF2 ethernet and PHY for BCM Cygnus SoC

Signed-off-by: Jiandong Zheng <jdzheng@broadcom.com>
Signed-off-by: Steve Rae <srae@broadcom.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>