[fdt] Use phandle as device location

[fdt] Use phandle as device location

Consumption of phandles will be in the form of locating a functional
device (e.g. a GPIO device, or an I2C device, or a reset controller)
by phandle, rather than locating the device tree node to which the
phandle refers.

Repurpose fdt_phandle() to obtain the phandle value (instead of
searching by phandle), and record this value as the bus location
within the generic device structure.

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