clk: renesas: r8a774c0: Correct parent clock of DU

According to the RZ/G Series, 2nd Generation Hardware Manual Rev 0.61,
the parent clock of the DU module clocks on RZ/G2E is S1D1.

Fixes: 906e0a4a ("clk: renesas: cpg-mssr: Add r8a774c0 support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Chris Paterson <chris.paterson2@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>