boards: arduino_nano_33_ble: define LED in DT

The board has a user LED attached to P1.09. This information was
hardcoded in board C files and enabled by default. This is something
applications need to do if needed (note that an always ON LED can incur
a considerable power consumption).

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>