Add XHCI support to powerpc64 GENERIC. This is useful to get input devices
supported on newer POWER hardware and in graphical VMs run on the same,
which are typically XHCI-only. The 32-bit GENERIC kernel, which
does not run on hardware made in the last decade and is unlikely to
encounter XHCI devices, is left unchanged.

PR: kern/224940
Submitted by: Gustavo Romero
MFC after: 1 week