lib/devicetree: Support 64 bit addresses for the initrd

The "linux,initrd-start" and "linux,initrd-end" properties encode the start
and end address of the initrd. The size of the address is encoded in the
root node #address-cells property and can be 1 cell (32 bits) or 2 cells
(64 bits). Add support for parsing a 64 bit address.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
[Fixed compilation for 32-bit arm with another cast.]
Signed-off-by: Andrew Jones <drjones@redhat.com>