dt-bindings: clock: marvell,pxa1908: Add syscon compatible to apmu

Add required syscon compatible and #power-domain-cells to the APMU
controller. This is required for the SoC's power domain controller as
the registers are shared.

Device tree bindings for said power domains are also added.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Duje Mihanović <duje@dujemihanovic.xyz>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>