cmake: dts: give each DTS_ROOT a vendor-prefixes.txt
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 <9d90ac4c7bf6a305f6bfd81a23c7859bc883380e@nordicsemi.no>

finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr