Add support for Ultra96 board

[sgherbst]

This largely depends on #23, since Ultra96 is a board that requires the PL clock to be sourced from the PS. Actually, ULTRA96 is already a board type, but it doesn't work yet, so this task likely involves updating the ULTRA96 class accordingly after #23 is resolved. https://github.com/sgherbst/anasymod/blob/fafaa0837388fac4883f0cd4e29de222ba5bced2/anasymod/fpga_boards/boards.py#L81-L91