doc: dt: mtd: partitions: add compatible property to "partitions" node

As noted here [1], there are potentially future conflicts if we try to
use MTD's "partitions" subnode to describe anything besides just the
fixed-in-the-device-tree partitions currently described in this
document. Particularly, there was a proposal to use this node for the
AFS parser too.

It can pose a (small) problem to try to differentiate the following
nodes:

	// using binding as currently specified
	partitions {
		#address-cells = <x>;
		#size-cells = <y>;
		partition@0 {
			...;
		};
	};

and

	// proposed future binding
	partitions {
		compatible = "arm,arm-flash-structure";
	};

It's especially difficult if other uses of this node start having
subnodes.

So, since the "partitions" node is new in v4.4, let's fixup the binding
before release so that it requires a compatible property, so it's much
clearer to distinguish. e.g.:

	// proposed
	partitions {
		compatible = "fixed-partitions";
		#address-cells = <x>;
		#size-cells = <y>;
		partition@0 {
			...;
		};
	};

[1] Subject: "mtd: create a partition type device tree binding"
    http://lkml.kernel.org/g/20151113220039.GA74382@google.com
    http://lists.infradead.org/pipermail/linux-mtd/2015-November/063355.html
    http://lists.infradead.org/pipermail/linux-mtd/2015-November/063364.html

Cc: Michal Suchanek <hramrach@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/mtd/partition.txt

@@ -6,7 +6,9 @@ used for what purposes, but which don't use an on-flash partition table such
 as RedBoot.
 
 The partition table should be a subnode of the mtd node and should be named
-'partitions'. Partitions are defined in subnodes of the partitions node.
+'partitions'. This node should have the following property:
+- compatible : (required) must be "fixed-partitions"
+Partitions are then defined in subnodes of the partitions node.
 
 For backwards compatibility partitions as direct subnodes of the mtd device are
 supported. This use is discouraged.
@@ -36,6 +38,7 @@ Examples:
 
 flash@0 {
 	partitions {
+		compatible = "fixed-partitions";
 		#address-cells = <1>;
 		#size-cells = <1>;
 
@@ -53,6 +56,7 @@ flash@0 {
 
 flash@1 {
 	partitions {
+		compatible = "fixed-partitions";
 		#address-cells = <1>;
 		#size-cells = <2>;
 
@@ -66,6 +70,7 @@ flash@1 {
 
 flash@2 {
 	partitions {
+		compatible = "fixed-partitions";
 		#address-cells = <2>;
 		#size-cells = <2>;
 

drivers/mtd/ofpart.c

@@ -55,6 +55,9 @@ static int parse_ofpart_partitions(struct mtd_info *master,
 			 master->name, mtd_node->full_name);
 		ofpart_node = mtd_node;
 		dedicated = false;
+	} else if (!of_device_is_compatible(ofpart_node, "fixed-partitions")) {
+		/* The 'partitions' subnode might be used by another parser */
+		return 0;
 	}
 
 	/* First count the subnodes */