dt-bindings: firmware: Add bindings for Tegra210 BPMP

The BPMP is a specific processor in Tegra210 chip, which is designed
for boot process handling, assisting in entering deep low power states
(suspend to ram), and offloading DRAM memory clock scaling on some
platforms.

Signed-off-by: Timo Alho <talho@nvidia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>