usb: UniPhier: support OF configuration

If CONFIG_OF_CONTROL is defined, search device tree nodes that are
compatible with "panasonic,uniphier-ehci" and take the base address
from their "reg" property.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Marek Vasut <marex@denx.de>