clk: renesas: r8a77995: Correct parent clock of DU

According to the R-Car Gen3 Hardware Manual Rev 1.00, the parent clock
of the DU module clocks on R-Car D3 is S1D1.

Fixes: d71e851d ("clk: renesas: cpg-mssr: Add R8A77995 support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>