Recent UEFI builds have moved the serial output from the (unpopulated) 4 pin header to the LS connector.
Try hooking up your UART to the UART1 pins on the LS connector.
Note that the UART1 pins are labeled UART3 in the HiKey hardware manual. LS UART0 and LS UART1 pins are connected to HI6220’s second and third UARTs hence both set of numbers can be correct at the same time ;-). Whatever number you use for the UART you need to connect to pins 11 and 13 on the LS connector.