pinctrl: sh-pfc: r8a7794: Add SSI pin groups

Add the SSI pin groups to the R8A7794 PFC driver.

[Sergei: fixed inconsistent alternate pin group naming, split SSI5/6 pin
groups into data/control ones, moved SSI7 data B group to its proper place,
fixed  pin names in  the comments to *_pins[], extended Cogent Embedded's
copyright, added the changelog, renamed the patch.]

Signed-off-by: Ryo Kataoka <ryo.kataoka.wt@renesas.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>