bcm2838_xhci: add module

bcm2838_xhci(4) is a shim for the XHCI controller on the Raspberry Pi 4B
SoC. It loads the controller's firmware before passing control to the
normal xhci(4) driver.

When xhci(4) is built as a module (and not in the kernel), bcm2838_xhci
is not built at all and the RPi4's XHCI controller won't attach due to
missing firmware.

To fix this, build a new module, bcm2838_xhci.ko, which depends on
xhci.ko. For the dependency to work correctly, also modify xhci to
provide the 'xhci' module in addition to the 'xhci_pci' module it
already provided.

Since bcm2838_xhci is specific to a quirk of the RPi4 SoC, only build
the module for AArch64.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1142