sh-pfc: r8a7779: Replace hardcoded pin numbers with RCAR_GP_PIN macro

Use the RCAR_GP_PIN macro to convert from the documentation pin number
space to the linear pinctrl space.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>