ARM: dts: imx7-colibri: fix PMIC voltages

Fix wrong voltage of PWR_EN_+V3.3 rail. The error had no noticeable
effect since no consumer explicitly requested a specific voltage.
Also use round voltages as it is common in other device trees.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>