ARM: dts: ux500: Break out DB8500 DTSI

ARM: dts: ux500: Break out DB8500 DTSI

The DB8500 exists in an enhanced variant named DB8520
for some machines. To clearly distinguish between the
different machines, create an explicit db8500.dtsi
and move the operating points (only known difference so
far) to that file, so we can add an explicit db8520.dtsi
after this.

Cc: Stephan Gerhold <stephan@gerhold.net>
Link: https://lore.kernel.org/r/20191126124738.77690-1-linus.walleij@linaro.org
Reviewed-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>