doc: dt: arm,pl172: add description of PL175 and PL176 controllers

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Joachim Eastwood <manabian@gmail.com>

Documentation/devicetree/bindings/memory-controllers/arm,pl172.txt

-* Device tree bindings for ARM PL172 MultiPort Memory Controller
+* Device tree bindings for ARM PL172/PL175/PL176 MultiPort Memory Controller
 
 Required properties:
 
-- compatible:		"arm,pl172", "arm,primecell"
+- compatible:		Must be "arm,primecell" and exactly one from
+			"arm,pl172", "arm,pl175" or "arm,pl176".
 
 - reg:			Must contains offset/length value for controller.
 
@@ -56,7 +57,8 @@ Optional child cs node config properties:
 
 - mpmc,extended-wait:	Enable extended wait.
 
-- mpmc,buffer-enable:	Enable write buffer.
+- mpmc,buffer-enable:	Enable write buffer, option is not supported by
+			PL175 and PL176 controllers.
 
 - mpmc,write-protect:	Enable write protect.