usb: add support for generic EHCI devices

This driver is meant to be used with any EHCI-compatible host
controller in case if there's no need for platform-specific
glue such as setup of controller or PHY's power mode via
GPIOs etc.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Stephen Warren <swarren@nvidia.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Marek Vasut <marex@denx.de>