xen/arm: Check if the device is available before using it

It's possible to have a device description in the DTS but the device is not
wired.

device_init must check if the device is available before doing anything with
it.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

xen/arch/arm/device.c

@@ -47,6 +47,9 @@ int __init device_init(struct dt_device_node *dev, enum device_type type,
 
     ASSERT(dev != NULL);
 
+    if ( !dt_device_is_available(dev) )
+        return  -ENODEV;
+
     for ( desc = _sdevice; desc != _edevice; desc++ )
     {
         if ( desc->type != type )