ARM: dts: imx6sl-evk: Rearrange the regulators node

It is not recommended to place regulators inside "simple-bus", so move
them out.

The motivation for doing this is to make it easier to adding new
regulators.

Signed-off-by: Marco Franchi <marcofrk@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>