fdt: fix fdtdec_get_pci_addr() for CONFIG_PHYS_64BIT

PCI addresses are always represented as 3 cells in DT. (one cell for bus
and device, and two cells for a 64-bit addres). This does not vary based
on either the physical address size of the CPU, nor any #address-cells
property in DT (or more precisely, #address-cells must be set to 3 in any
PCIe controller's node).

Fix fdtdec_get_pci_addr() to use conversion functions that operate on
(fixed) cell-sized data rather than (varying) physical-address-sized
data, so that the function works on 64-bit systems.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Thierry Reding <treding@nvidia.com>