arm64: dts: renesas: r8a774a1: Replace clock magic numbers

Now that include/dt-bindings/clock/r8a774a1-cpg-mssr.h is in Linus'
master branch we can replace clock related magic numbers with the
corresponding labels.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
[simon: corrected whitespace]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>