ci20: Update default uart to ttyS4

ttyS4 is the dedicated uart header on the ci20.
CONFIG_CONS_INDEX starts from 1 so 5 points to ttyS4.

Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
ci20: Change kernel bootargs to default to ttyS4

ttyS4 goes on the dedicated white UART header on the CI20.

Linux Kernel cannot handle serial output during boot to multiple
ttySx sources.

Default to ttyS4.

Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>