boards: raytac_mdbt53_db_40_nrf5340: Add HW flow control pins

Add HW flow control pins for raytac_mdbt53_db_40_nrf5340 board.
Use the same pins as the cpunet has been assigned, as the RX / TX lines
was also using the same as cpunet.
Remove bias-pull-up as uart1 using same pins on cpunet does not have
this either.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>