pinctrl: sh-pfc: r8a7791: Add missing DVC_MUTE signal

The R8A7791 PFC driver  was apparently based on the preliminary revisions
of  the user's  manual, which  omitted the DVC_MUTE signal  altogether in
the PFC section. The modern manual has the signal described,  so just add
the necassary data to the driver...

Fixes: 50884519 ("pinctrl: sh-pfc: r8a7791 PFC support")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>