hw/mips/cps: Do not allow use without input clock

Now than all QOM users provides the input clock, do not allow
using a CPS without input clock connected.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20201012095804.3335117-21-f4bug@amsat.org>

hw/mips/cps.c

@@ -74,6 +74,11 @@ static void mips_cps_realize(DeviceState *dev, Error **errp)
     bool itu_present = false;
     bool saar_present = false;
 
+    if (!clock_get(s->clock)) {
+        error_setg(errp, "CPS input clock is not connected to an output clock");
+        return;
+    }
+
     for (i = 0; i < s->num_vp; i++) {
         cpu = MIPS_CPU(object_new(s->cpu_type));