Willena

After having spent a lot of time searching for the solution, I finally found it.
 
UART1 is not enabled by default. But referenced in the device-tree. It was as simple as changing some option in the dts.
After figuring out the symbol of UART1 in the symbol section of the dts
__symbols__ { ... uart0 = "/serial@ff110000"; uart1 = "/serial@ff120000"; uart2 = "/serial@ff130000"; ...  
Further in the file we can find more information on the definition of uart1 (also known with symnol serial@ff120000)
serial@ff120000 { compatible = "rockchip,rk3328-uart\0snps,dw-apb-uart"; reg = < 0x00 0xff120000 0x00 0x100 >; interrupts = < 0x00 0x38 0x04 >; clocks = < 0x02 0x27 0x02 0xd3 >; clock-names = "baudclk\0apb_pclk"; dmas = < 0x12 0x04 0x12 0x05 >; dma-names = "tx\0rx"; pinctrl-names = "default"; pinctrl-0 = < 0x25 0x26 0x27 >; reg-io-width = < 0x04 >; reg-shift = < 0x02 >; status = "disabled"; phandle = < 0x85 >; }; Notice the disabled value in the status field.
Changing the status from 'disabled' to 'okay' did the trick.
After a restart UART1 was available on /dev/ttyS1 \0/