review vivado cdc analysis (report_cdc)

[jordens]

Vivado flags a bunch of "unknown" (~80 from dac_refclk to each jesd phy_tx_clk and 16 from dac_refclk to sys_clk) and "unsafe" (1-3 on each clock domain, presumably comb logic in front of the ARS) CDCs on Sayma.

"Clock enable controlled CDC structure detected" "Combinational logic detected before a synchronizer" "Multi-bit synchronized with ASYNC_REG property"

Maybe related: #58 #103