cmake: dts: give each DTS_ROOT a vendor-prefixes.txt

Now that the infrastructure we need is in place, we can allow every
element of DTS_ROOT to have its own dts/bindings/vendor-prefixes.txt
file instead of just special-casing ZEPHYR_BASE (which is always in
DTS_ROOT).

This treats all modules equally and eases out of tree management of
downstream-specific vendor prefixes.

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