dt-bindings: firmware: 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>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
[treding@nvidia.com: renamed bpmp-i2c to i2c as per Rob]
Signed-off-by: Thierry Reding <treding@nvidia.com>