arm64: dts: zii-ultra: fix 12V_MAIN voltage

As this is a fixed regulator on the board there was no harm in the wrong
voltage being specified, apart from a confusing reporting to userspace.

Fixes: 4a13b3be

 ("arm64: dts: imx: add Zii Ultra board support")
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>