ARM: sun8i: A23: Add missing msgbox gate

Even though it's not mentionned in the A23 user manual, the A23 has a gate
for the AHB1 clock to the msgbox IP. Add it to the clock-indices.

Reported-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>