device/device_util: Add function to determine bridge state

Add a method to get the state of a bridge device.
Return true if at least one enabled device on the secondary
bus is found.

Useful to disable non hotplugable bridges without any devices attached.

Change-Id: Ic8fe539d233031d4d177b03dd2c03edb5ab8c88d
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/19817
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philippe Mathieu-Daudé <philippe.mathieu.daude@gmail.com>
Reviewed-by: Martin Roth <martinroth@google.com>

src/include/device/device.h

@@ -190,6 +190,7 @@ u32 dev_path_encode(device_t dev);
 const char *bus_path(struct bus *bus);
 void dev_set_enabled(device_t dev, int enable);
 void disable_children(struct bus *bus);
+bool dev_is_active_bridge(device_t dev);
 
 /* Option ROM helper functions */
 void run_bios(struct device *dev, unsigned long addr);