ARM: dts: ux500: Move ab8500 nodes to ste-ab8500.dtsi

Some Ux500 devices use the newer AB8505 PMIC instead of AB8500.
Although they are very similar, there are subtle differences
like the number of regulators or the available GPIO pins.

At the moment, ste-dbx5x0.dtsi always configures the AB8500 PMIC.
To support devices with AB8505, it is necessary to split the
AB8500-specific parts into a separate .dtsi file. Boards can then
select the PMIC by including either ste-ab8500.dtsi or ste-ab8505.dtsi.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>