hw/arm_boot.c: Support DTBs which use 64 bit addresses

hw/arm_boot.c: Support DTBs which use 64 bit addresses

Support the case where the device tree blob specifies that
#address-cells and #size-cells are greater than 1. (This
is needed for device trees which can handle 64 bit physical
addresses and thus total RAM sizes over 4GB.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter A. G. Crosthwaite <peter.crosthwaite@petalogix.com>