ARM: dts: imx: Replace "simple-bus" with "simple-mfd" for anatop

Replace "simple-bus" with "simple-mfd" for anatop node in order
to fix the following build warnings with W=1:

arch/arm/boot/dts/imx6sx.dtsi:603.31-616.7: Warning (simple_bus_reg): /soc/aips-bus@2000000/anatop@20c8000/regulator-1p1: missing or empty reg/ranges property
arch/arm/boot/dts/imx6sx.dtsi:618.31-631.7: Warning (simple_bus_reg): /soc/aips-bus@2000000/anatop@20c8000/regulator-3p0: missing or empty reg/ranges property
arch/arm/boot/dts/imx6sx.dtsi:633.31-646.7: Warning (simple_bus_reg): /soc/aips-bus@2000000/anatop@20c8000/regulator-2p5: missing or empty reg/ranges property
arch/arm/boot/dts/imx6sx.dtsi:648.32-663.7: Warning (simple_bus_reg): /soc/aips-bus@2000000/anatop@20c8000/regulator-vddcore: missing or empty reg/ranges property
arch/arm/boot/dts/imx6sx.dtsi:665.33-679.7: Warning (simple_bus_reg): /soc/aips-bus@2000000/anatop@20c8000/regulator-vddpcie: missing or empty reg/ranges property
arch/arm/boot/dts/imx6sx.dtsi:681.31-696.7: Warning (simple_bus_reg): /soc/aips-bus@2000000/anatop@20c8000/regulator-vddsoc: missing or empty reg/ranges property

Based on a patch from Marco Felsch for the imx6qdl.dtsi.

Cc: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>