dm: core: regmap: Avoid multiple calls to ofnode_get_parent()

dm: core: regmap: Avoid multiple calls to ofnode_get_parent()

Until a live tree is used in U-Boot proper after relocation, use of
ofnode_get_parent() will trigger a call to the very expensive
fdt_parent_offset(), 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 node.

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