[fdt] Allow for the existence of multiple device trees

[fdt] Allow for the existence of multiple device trees

When running on a platform that uses FDT as its hardware description
mechanism, we are likely to have multiple device tree structures.  At
a minimum, there will be the device tree passed to us from the
previous boot stage (e.g. OpenSBI), and the device tree that we
construct to be passed to the booted operating system.

Update the internal FDT API to include an FDT pointer in all function
parameter lists.

Signed-off-by: Michael Brown <mcb30@ipxe.org>