sh-pfc: r8a7778: Fix outdated GPIO_FN comments

Function GPIOs have been removed, remove comments that refer to them.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>