arm64: dts: renesas: r8a77965: use r8a77965-sysc binding definitions

Replace the hardcoded power domain indices by R8A77965_PD_* symbols.
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
[simon: dropped hunk to include r8a77965-sysc.h which is already present]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>