ARM: multi_v7_defconfig: Enable BRCMSTB networking

Enable the required drivers and subsystems to bring-up networking on
Broadcom's Set-top Box SoCs: BCMGENET and SF2. These two drivers both
have proper dependencies on their internal PHY and MDIO drivers.

Acked-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>