Add a macro to refer to another pin group with a different name.

This will be used to rename wrongly-named pin groups, while retaining
backwards compatibility with old DTBs.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>