firmware

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>