dt-bindings: allow child nodes inside the Tegra BPMP

The BPMP implements some services which must be represented by separate
nodes. For example, it can provide access to certain I2C controllers, and
the I2C bindings represent each I2C controller as a device tree node.
Update the binding to describe how the BPMP supports this.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>

doc/device-tree-bindings/firmware/nvidia,tegra186-bpmp.txt

@@ -38,6 +38,24 @@ implemented by this node:
 - .../reset/reset.txt
 - <dt-bindings/reset/tegra186-reset.h>
 
+The BPMP implements some services which must be represented by separate nodes.
+For example, it can provide access to certain I2C controllers, and the I2C
+bindings represent each I2C controller as a device tree node. Such nodes should
+be nested directly inside the main BPMP node.
+
+Software can determine whether a child node of the BPMP node represents a device
+by checking for a compatible property. Any node with a compatible property
+represents a device that can be instantiated. Nodes without a compatible
+property may be used to provide configuration information regarding the BPMP
+itself, although no such configuration nodes are currently defined by this
+binding.
+
+The BPMP firmware defines no single global name-/numbering-space for such
+services. Put another way, the numbering scheme for I2C buses is distinct from
+the numbering scheme for any other service the BPMP may provide (e.g. a future
+hypothetical SPI bus service). As such, child device nodes will have no reg
+property, and the BPMP node will have no #address-cells or #size-cells property.
+
 The shared memory bindings for BPMP
 -----------------------------------
 
@@ -78,4 +96,9 @@ bpmp {
 	#clock-cells = <1>;
 	#power-domain-cells = <1>;
 	#reset-cells = <1>;
+
+	i2c {
+		compatible = "...";
+		...
+	};
 };