dt-bindings: power: r8a779g0: Add missing A3DUL power domain

Add the power domain index for the A3DUL domain, as described in the
R-Car V4H Series Hardware User's Manual Rev. 0.51 and later.

Signed-off-by: Tam Nguyen <tam.nguyen.xa@renesas.com>
[geert: Manual reference]
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/d61cf54b58629a76e007b9505dac7c2beb3b30db.1669740926.git.geert+renesas@glider.be