dts: bindings: wifi: split nrf700x coex and wifi models

The nrf7000, nrf7001 and nrf7002 expose a coex hardware interface
which is independent from the wifi/control interface (spi and
control pins)

These interfaces where previously combined into a single model. This
is incompatible with the actual usage of the interfaces where one
core may interact only with the coex interface, and another with the
wifi/control interface.

This commit moves the coex interface, commonly described in
"nordic,nrf70-coex.yaml", out of the wifi/control models
"nrf700<variant>-<bus>.yaml" to its own models
"nrf700<variant>-coex.yaml"

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>