dm: core: fdtaddr: Avoid multiple calls to fdt_parent_offset()

dm: core: fdtaddr: Avoid multiple calls to fdt_parent_offset()

Use of fdt_parent_offset() is very expensive as detailed by the function
documentation:

  NOTE: This function is expensive, as it must scan the device tree
  structure from the start to nodeoffset, *twice*.

Re-use the returned value from a single call instead of having to make
multiple calls for same nodeoffset.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Simon Glass <sjg@chromium.org>