Add function to encode device path into integer

This function will encode the device path into 3
bytes of a dword which can be saved for debug.

It will be used by subsequent commit to store the
current device into CMOS for debugging BIOS hangs.

Change-Id: I3a5155ea53c8d280806e610a0f8998dbabe15f3c
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/58103
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/4228
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>

src/include/device/device.h

@@ -154,6 +154,7 @@ void enumerate_static_device(void);
 void enumerate_static_devices(void);
 const char *dev_name(device_t dev);
 const char *dev_path(device_t dev);
+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);