ARM: bcm2835_defconfig: Enable serial & ethernet USB gadget support

In order to use the serial and ethernet USB gadget support on
Raspberry Zero, we also need to enable the PHY driver, kernel module
and OTG support.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Eric Anholt <eric@anholt.net>