The RFSoC 2x2 board files can be found on the Xilinx Board Store
You can install the board files from the Vivado Tcl prompt with the following commands:
xhub::refresh_catalog [xhub::get_xstores xilinx_board_store] xhub::install [xhub::get_xitems xilinx.com:xilinx_board_store:rfsoc2x2:1.1]
If Vivado is open you you need to restart it before you create a new project.
The next time you create a New Project, you will have an option to select the RFSoC 2x2 board as shown below.