Exynos: Move down common USB configuration

USB is a pretty common feature on exynos 5 board, so it seems sensible
to configure it directly from exynos5-common. As a side-effect this
makes USB available from u-boot on exynos 5420 based boards.

While there enable support for common USB ethernet cards to make it more
likely the default config allows booting for network and enable XHCI on
SMDK5420 which has it defined in the dts but not in its config.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>