ARM: shmobile: rcar-gen2: Drop comma after OF match table sentinel

It does not make sense to have a comma after a sentinel, as any new
elements must be added before the sentinel.

Add a comment to clarify the purpose of the empty element.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/baaa4df6284401eb126573eb1c8ea5a88705cc37.1646311858.git.geert+renesas@glider.be