boards: renesas: da14695_dk_usb: added mikrobus node labels

Added mikrobus_header, mikrobus_i2c, mikrobus_spi and mikrobus_serial
node labels to da14695_dk_usb device tree board definition, allowing
compatible shield boards to be used. Also fixed minor issues with
pin assignment and header labelling.

Signed-off-by: Ian Morris <ian.d.morris@outlook.com>