usb: xhci-dwc3: Support role switch default role

When the device tree indicates support for role switching through
the "usb-role-switch" property, take the "role-switch-default-mode"
property into account when deciding which role to put the
controller into.

This makes USB devices work on Apple M1 systems where the device
tree may include a "dr_mode" property that is set to "otg", but
where we need to put the controller into "host" mode to see
devices connected to the type-C ports.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>