drivers: wifi: nrfwifi: determine shared iovdd and bucken from dt

Currently a user, or a shield definition, must select a Kconfig to
signal that the iovdd_ctrl and bucken GPIOs are connected to the same
pin. We can instead check this in the driver by simply checking if
the iovdd_ctrl_spec and bucken_spec are the same pin.

Update the nrfwifi driver and remove the redundant
NRF_WIFI_COMBINED_BUCKEN_IOVDD_GPIO config.

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