chore: add clocks for accum_fifo impl test

rtl/accum/tests/test_timing.xdc

@@ -0,0 +1,10 @@
+# Primary clocks
+create_clock -name eth_clk -period 8.000  [get_ports eth_clk_in]
+create_clock -name acc_clk -period 15.385  [get_ports acc_clk_in]
+
+
+# Asynchronous clock groups
+
+set_clock_groups -name ASYNC_ETH_ACC -asynchronous \
+    -group [get_clocks eth_clk] \
+    -group [get_clocks acc_clk]