Documentation: bindings: add compatible specific to pre v1.0 SCPI protocols

This patch adds specific compatible to support all the unversioned SCPI
protocols prior to v1.0 release. This will be applicable for all the
implementations using draft versions or modified versions of those
draft vesrions of SCPI protocol.

Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

Documentation/devicetree/bindings/arm/arm,scpi.txt

@@ -7,7 +7,10 @@ by Linux to initiate various system control and power operations.
 
 Required properties:
 
-- compatible : should be "arm,scpi"
+- compatible : should be
+	* "arm,scpi" : For implementations complying to SCPI v1.0 or above
+	* "arm,scpi-pre-1.0" : For implementations complying to all
+		unversioned releases prior to SCPI v1.0
 - mboxes: List of phandle and mailbox channel specifiers
 	  All the channels reserved by remote SCP firmware for use by
 	  SCPI message protocol should be specified in any order