IOVDD is supplied by 1.8V, fix the referenced regulator.

Fixes: d8f9d812 ("arm64: dts: imx8mp: Add analog audio output on i.MX8MP TQMa8MPxL/MBa8MPxL")
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>