ARM: exynos_defconfig: enable USB gadget support

Enable USB gadget support without support for any specific gadgets to
more easily catch cases where a devices dts doesn't specify the usb
controllers dr_mode while it should.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>