ARM: dts: sun8i: a711: Reinstate the PMIC compatible

When we added the regulator support in commit 90c5d7cd ("ARM: dts:
sun8i: a711: Add regulator support"), we also dropped the PMIC's
compatible. Since it's not in the PMIC DTSI, unlike most other PMIC
DTSI, it obviously wasn't probing anymore.

Re-add it so that everything works again.

Fixes: 90c5d7cd

 ("ARM: dts: sun8i: a711: Add regulator support")
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>