we have the "default" tree, then the "nvme" tree which adds
nvme support.

the "xhci" tree adds xhci patches, *and* nvme patches.

riku suggested that i rename it accordingly, and his advice
was quite correct, indeed wise.

this will reduce confusion for contributors, including *myself*.

Signed-off-by: Leah Rowe <leah@libreboot.org>