omap-common: Common function to display die id, replacing omap3-specific version

This introduces omap_die_id_display to display the full die id.
There is no need to store it in an environment variable, that no boot script
is using anyway.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/board/technexion/tao3530/tao3530.c b/board/technexion/tao3530/tao3530.c
index 744ff44..d51b5d9 100644
--- a/board/technexion/tao3530/tao3530.c
+++ b/board/technexion/tao3530/tao3530.c
@@ -160,7 +160,7 @@
 		puts("Unknown board revision\n");
 	}
 
-	dieid_num_r();
+	omap_die_id_display();
 
 	return 0;
 }