dt-bindings: power: Add r8a774c0 SYSC power domain definitions

This patch adds power domain indices for RZ/G2E.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>