Add USB_EHCI_EXYNOS to multi_v7_defconfig

Currently we enable Exynos devices in the multi v7 defconfig, however, when
testing on my ODROID-U3, I noticed that USB was not working.  Enabling this
option causes USB to work, which enables networking support as well since the
ODROID-U3 has networking on the USB bus.

[arnd] Support for odroid-u3 was added in 3.10, so it would be nice to
backport this fix at least that far.

Signed-off-by: Steev Klimaszewski <steev@gentoo.org>
Cc: stable@vger.kernel.org # 3.10
Signed-off-by: Arnd Bergmann <arnd@arndb.de>