boards: arm: ubx_evkninab{3,4}: Update docs to use DEVICE_DT_GET

Update docs for boards ubx_evkninab3_nrf52840 and ubx_evkninab4_nrf52833
to use DEVICE_DT_GET as we phase out "label" usage and thus the
device_get_binding() reference wouldn't work.

Signed-off-by: Kumar Gala <galak@kernel.org>