Add a helper routine to depth-search the device tree for a node with a
matching 'compatible' property.  This probably has a short half-life (as
do most of the fdt_ functions), but it helps solve some near-term needs
until we work out the larger problems of device instantiation order
versus the order of things in the fdt data.
Make it possible to access the ocotp registers before the ocotp device
is attached, by establishing a temporary mapping of the registers when
necessary.  This is a temporary measure to keep progress moving; in the
long run we need better control over the order in which devices attach
(better than "the order they appear in the fdt dts source").