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

Now that include/dt-bindings/power/r8a774a1-sysc.h is in Linus'
master branch we can replace power 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>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>