hw/arm_sysctl: Implement SYS_CFG_MUXFPGA writes as a no-op

SYS_CFG_MUXFPGA allows the guest to select whether the
video output should come from the motherboard's LCD
controller or the daughterboard's one. Since QEMU doesn't
currently support selecting the video output like this,
implement as a no-op, so Linux doesn't complain about
the register not being implemented.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

hw/arm_sysctl.c

@@ -272,6 +272,17 @@ static bool vexpress_cfgctrl_write(arm_sysctl_state *s, unsigned int dcc,
     }
 
     switch (function) {
+    case SYS_CFG_MUXFPGA:
+        if (site == SYS_CFG_SITE_MB && device == 0) {
+            /* Select whether video output comes from motherboard
+             * or daughterboard: log and ignore as QEMU doesn't
+             * support this.
+             */
+            qemu_log_mask(LOG_UNIMP, "arm_sysctl: selection of video output "
+                          "not supported, ignoring\n");
+            return true;
+        }
+        break;
     case SYS_CFG_SHUTDOWN:
         if (site == SYS_CFG_SITE_MB && device == 0) {
             qemu_system_shutdown_request();