ARM: dts: imx6: replace simple-bus by simple-mfd for anatop

Replace it to fix the following DTC warnings:

arch/arm/boot/dts/imx6qdl.dtsi:702.19-715.7: Warning (simple_bus_reg): /soc/aips-bus@2000000/anatop@20c8000/regulator-1p1: missing or empty reg/ranges property
arch/arm/boot/dts/imx6qdl.dtsi:717.19-730.7: Warning (simple_bus_reg): /soc/aips-bus@2000000/anatop@20c8000/regulator-3p0: missing or empty reg/ranges property
arch/arm/boot/dts/imx6qdl.dtsi:732.19-745.7: Warning (simple_bus_reg): /soc/aips-bus@2000000/anatop@20c8000/regulator-2p5: missing or empty reg/ranges property
arch/arm/boot/dts/imx6qdl.dtsi:747.32-762.7: Warning (simple_bus_reg): /soc/aips-bus@2000000/anatop@20c8000/regulator-vddcore: missing or empty reg/ranges property
arch/arm/boot/dts/imx6qdl.dtsi:764.29-779.7: Warning (simple_bus_reg): /soc/aips-bus@2000000/anatop@20c8000/regulator-vddpu: missing or empty reg/ranges property
arch/arm/boot/dts/imx6qdl.dtsi:781.31-796.7: Warning (simple_bus_reg): /soc/aips-bus@2000000/anatop@20c8000/regulator-vddsoc: missing or empty reg/ranges property

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>