boards: arm: nrf52840_pca10090: add a GPIO reset line

Let one of the MCU interface pins be configured to act
as a reset line. This mitigates the lack of a connection
between the nRF9160 and nRF52840 PINRESET.
Minor refactoring and updated comments.

Signed-off-by: Emanuele Di Santo <emdi@nordicsemi.no>