pinctrl: sh-pfc: Add SH_PFC_PIN_GROUP_ALIAS()

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>