pinctrl: sh-pfc: r8a7794: Fix GP2[29] muxing

GP2[29] muxing is controlled by 2-bit IP6[3:2] field, yet only 3 values
are listed instead of 4...

[Sergei: fixed up the formatting, renamed, added the changelog.]

Signed-off-by: Andrey Gusakov <andrey.gusakov@cogentembedded.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>