clk: renesas: r8a77970: Add LVDS clock

I seem to have omitted the LVDS clock from the R8A77970 CPG/MSSR support
patch for some reason -- add it back...

Based on the original (and large) patch by Daisuke Matsushita
<daisuke.matsushita.ns@hitachi.com>.

Fixes: 8d46e28f

 ("clk: renesas: cpg-mssr: Add R8A77970 support")
Signed-off-by: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>