ARM: dts: r8a77470: Use r8a77470-cpg-mssr binding definitions

Replace the hardcoded clock indices by R8A77470_CLK_* symbols.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>