arm: apple: Enable PCIe USB controller

Some Apple Silicon machines have a PCIe XHCI controller in additon
to the DWC3 controllers integrated on the SoC.  On the Mac mini
the Type-A ports are handled by this PCIe controller.  Enabling
it allows the use of these ports in U-Boot.

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