Bluetooth: hci: h4: use GPIO reset for nrf52840_pca10090 controllers

This patch adds support to the nRF9160 for using a dedicated
GPIO pin to reset controllers running on nrf52840_pca10090.

It resets the controller before opening the H4 device, and it
delays the controller from booting until all bytes traveling
to the host have been received and drained from the UART,
thus ensuring that communication can begin from a clean state.

Signed-off-by: Emanuele Di Santo <emdi@nordicsemi.no>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>