arm: bcm7xxx: Convert to DM_SERIAL

Remove ns16550 configuration from header files.  Document
DM_SERIAL-required prior stage device tree configuration.
diff --git a/doc/README.bcm7xxx b/doc/README.bcm7xxx
index 9b5eae4..6839da5 100644
--- a/doc/README.bcm7xxx
+++ b/doc/README.bcm7xxx
@@ -31,6 +31,12 @@
 Run
 ===
 
+To tell U-Boot which serial port to use for its console, set the
+"stdout-path" property in the "/chosen" node of the BOLT-generated
+device tree.  For example:
+
+BOLT> dt add prop chosen stdout-path s serial0:115200n8
+
 Flash the u-boot binary into board storage, then invoke it from BOLT.
 For example: