boards: nrf9160dk: Switch to sda-gpios, scl-gpios DT properties

This is a follow-up to commit 763e73d7.

Switch to the new properties for specifying SDA and SCL pins,
as the old ones (sda-pin and scl-pin) are now deprecated.

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