net: move Broadcom SF2 driver to Kconfig

move to Kconfig:
	CONFIG_BCM_SF2_ETH
	CONFIG_BCM_SF2_ETH_DEFAULT_PORT
	CONFIG_BCM_SF2_ETH_GMAC

Also modified defconfigs of all platforms that use these configs.

Signed-off-by: Suji Velupillai <suji.velupillai@broadcom.com>
Tested-by: Suji Velupillai <suji.velupillai@broadcom.com>
Reviewed-by: JD Zheng <jiandong.zheng@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Steve Rae <steve.rae@raedomain.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>