grub xhci support

see:

https://github.com/9elements/grub/commits/xhci-module-upstreaming-squash_v4/

grub only supports xhci on bios/uefi targets, but not coreboot.
some newer machines don't have ps/2 controllers, and boot in a
way where ehci isn't available at startup; the controller can't
be used by ehci code, there must be xhci support.

the code is from Patrick Rudolph working on behalf of 9elements.
the code was also sent here for review:

https://lists.gnu.org/archive/html/grub-devel/2020-12/msg00111.html

however, upstream never merged these patches. libreboot will have
to maintain these from now on. the patches have been rebased for
use with grub 2.12.

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

config/grub/modules.list

@@ -36,6 +36,7 @@ crypto \
 cryptodisk \
 diskfilter \
 echo \
+xhci \
 ehci \
 eval \
 exfat \