config: More USB config options for bcm2709_defconfig (#1805)

Bring bcm2709_defconfig up to the level of bcmrpi_defconfig with
respect to USB support. This is prompted by the introduction of CM3,
which makes gadget mode on 2709/2710 a useful option.

Note that the DWC2 driver is not loaded by default and must be enabled
using a DT overlay or custom .dtb.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>