usb: add CONFIG_USB_UHCI_HCD in Kconfig

There is no UHCI driver entry in Kconfig for now, but we have some
UHCI drivers, for example, LEON.  This is a placeholder in case we
want to move them to Kconfig in the future.

The help message was copied from Linux.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>