ARM: dts: imx28-apx4devkit: Fix the regulator description

The regulator should not be placed under simple-bus.

Remove it from simple-bus to fix the following dt-schema warnings:

'regulators' does not match '^([a-z][a-z0-9\\-]+-bus|bus|localbus|soc|axi|ahb|apb)(@.+)?$'
regulators: #size-cells:0:0: 0 is not one of [1, 2]
regulators: regulator@0:reg:0: [0] is too short
regulators: 'ranges' is a required property

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>