cmake: modules: dts: extract preprocessing helper extension

Tighten up the interface boundaries by adding an extension function
that separates *how* we preprocess the DTS from *what* DTS files we
are preprocessing. This involves a functional change to the pre_dt
module.

This is useful cleanup but is also groundwork for relying on this
helper function when adding system devicetree support.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>