Add the FDT table GUID. This is used to pass the device tree blob from UEFI
to the loader in a similar way to the ACPI tables.

This will be used on arm64 but is not specific to the architecture.

Sponsored by:	The FreeBSD Foundation


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274439 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f