Handle Raspberry Pi 4 xhci firmware loading.

The newer hardware revisions of the Raspberry Pi 4 removed the ability of
the VIA VL805 xhci controller to load its own firmware. Instead the
firmware must be installed at the appropriate time by the VideoCore
coprocessor.

Submitted by: Robert Crowston <crowston_protonmail.com>
Differential Revision: https://reviews.freebsd.org/D25261