n9: initialize only UART3

On N9 only one UART3 pins are used as serial port, so change init
to initialize only UART3.

Signed-off-by: Filip Matijević <filip.matijevic.pz@gmail.com>
n9: add config options as reported by mer_verify_kernel_config

Signed-off-by: Filip Matijević <filip.matijevic.pz@gmail.com>