boards: nrf9160dk: Add DT node for I/O expander available in v0.14.0+

Add devicetree node representing the PCAL6408A I2C-based I/O expander
that is available in nRF9160 DK v0.14.0 or later.
Provide also .dtsi files that can be used in applications to simplify
switching to interfacing onboard LEDs and buttons through this expander
instead of SoC pins.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>