pinctrl: sh-pfc: checker: Add helper for safe name comparison

Add a helper to check if two strings are identical, skipping NULL
pointers.  This simplifies callers.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20200110131927.1029-4-geert+renesas@glider.be