support absence of gateware RTIO clock selection mux

src/gateware/zc706.py

@@ -83,6 +83,7 @@ class ZC706(SoCCore):
 
         self.submodules.rtio_crg = RTIOCRG(self.platform, self.ps7.cd_sys.clk)
         self.csr_devices.append("rtio_crg")
+        self.rustc_cfg["has_rtio_crg_clock_sel"] = None
         self.platform.add_period_constraint(self.rtio_crg.cd_rtio.clk, 8.)
         self.platform.add_false_path_constraints(
             self.ps7.cd_sys.clk,

src/runtime/src/main.rs

@@ -108,6 +108,7 @@ fn init_rtio(timer: &mut GlobalTimer, cfg: &Config) {
     loop {
         unsafe {
             pl::csr::rtio_crg::pll_reset_write(1);
+            #[cfg(has_rtio_crg_clock_sel)]
             pl::csr::rtio_crg::clock_sel_write(clock_sel);
             pl::csr::rtio_crg::pll_reset_write(0);
         }