fdt: Add function to locate an array in the device tree

fdtdec_locate_array() locates an integer array but does not copy it. This
saves the caller having to allocated wasted space.

Access to array elements should be through the fdt32_to_cpu() macro.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>