usb: uniphier: remove UniPhier xHCI driver and select DM_USB

This driver has not been converted to Driver Model, and it is an
obstacle to migrate other block device drivers.  Remove it for now.

The UniPhier SoCs already use a DM-based EHCI driver, so now
ARCH_UNIPHIER can select DM_USB.

These two changes must be done atomically because removing the
legacy driver causes a build error.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Marek Vasut <marex@denx.de>