pinctrl: sh-pfc: r8a7792: Fix vin1_data18_b pin group

The vin1_data18_b pin group itself is present, but it is not listed in
the VIN1 pin group array, and thus cannot be selected.

Fixes: 7dd74bb1 ("pinctrl: sh-pfc: r8a7792: Add VIN pin groups")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>