dm: usb: Refactor EHCI init

Move the bulk of the code in usb_lowlevel_init() into a separate function
which will also be used by driver model. Keep the CONFIG options out of
this function by providing a tweak flag for Faraday. We need to avoid using
CONFIG options in driver model code where possible, since it makes it
impossible to use multiple controllers in that code where they have
different options.

The CONFIG_EHCI_HCD_INIT_AFTER_RESET option is also kept out of the
common init function. With driver model the controller will be able to
perform this extra init itself after registering with the EHCI layer.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>