Devicetree: Expand on ARM Primecell binding documentation

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

Documentation/devicetree/bindings/arm/primecell.txt

@@ -6,7 +6,9 @@ driver matching.
 
 Required properties:
 
-- compatible : should be a specific value for peripheral and "arm,primecell"
+- compatible : should be a specific name for the peripheral and
+               "arm,primecell".  The specific name will match the ARM
+               engineering name for the logic block in the form: "arm,pl???"
 
 Optional properties: