Remove deprecated <= assignments

Starting from cocotb v1.6 the use of <= syntax has been deprecated. This
commit replaces all use of this syntax with the ``.value =`` syntax.

tests/axi/test_axi.py

@@ -73,10 +73,10 @@ class TB:
         self.dut.rst.setimmediatevalue(0)
         await RisingEdge(self.dut.clk)
         await RisingEdge(self.dut.clk)
-        self.dut.rst <= 1
+        self.dut.rst.value = 1
         await RisingEdge(self.dut.clk)
         await RisingEdge(self.dut.clk)
-        self.dut.rst <= 0
+        self.dut.rst.value = 0
         await RisingEdge(self.dut.clk)
         await RisingEdge(self.dut.clk)