Each folder contains a .dtb file, which is the compiled device tree itself, plus a .dts with the original source and a .gpio file showing the GPIO pin mappings.

Generic

(The generic device tree I will use in my project, suitable for many occasions)[./generic]

Has 2x UART (plus the console uart), 2x I2C. 1xSPI
12 unassigned GPIO pins

Serial

(Four UART interfaces and Four I2C interfaces)[./serial]

Has 4x UART (plus the console uart), 4x I2C
UART3 has RTC/CTS pins available too
10 unassigned GPIO pins

SPI

(SPI plus Serial interfaces)[./spi]

Has 3x UART (plus the console uart), 3x I2C. 1xSPI
8 unassigned GPIO pins

SunXI

(Vanilla, unpopulated, upstream)[./sunxi]

Has the console uart, nothing more
26 unassigned GPIO pins!

AllWinner Nezha

(DO NOT USE)[./allwinner-nezha]

Included for completeness, this is the default device tree you get with the Ubuntu image.
Has the console uart, 1x I2C. 1xSPI
15 unassigned GPIO pins, 3 pre-assigned to pinctl