xhci-platform: Change compatible string from xhci-platform to generic-xhci

This brings the xhci-platform bindings in sync with what we've done for
the ohci- and ehci-platform drivers. As discussed there using platform as a
postfix is a bit weird as the platform bus is a Linux specific thing and
the bindings are supposed to be OS agnostic.

Note that the old xhci-platform compatible string is kept around for, well,
compatibility reasons.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>