Attached teardown of the NES Classic
The CPU (Allwinner R16 aka A33) has 5 UART ports.
It is UART. As pointed out on this website:
There are PB0/PB1 (for mainline) and PF2/PF4 (for stock firmware) on SIDE-A.
If you want to use the PF2/PF4 pins using the mainline U-Boot, you'll need to change CONS_INDEX to 1, and set UART0_PORT_F in menuconfig.