sh-pfc: r8a7791: fix CAN pin groups

I had made last-minute changes before submitting the patch "sh-pfc: r8a7791:
add CAN pin groups"; now I'm seeing that they weren't complete: I had missed
update to the pin group names in pin[01]_groups[]. Drop the "_a" suffixes there.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>