dt-bindings: firmware: Document Tegra264 BPMP

While the BPMP found on Tegra264 is similar to the versions found on
previous chips and should be backwards-compatible, some changes could
eventually be needed. Anticipate such changes and introduce a chip-
specific compatible string.

Acked-by: Rob Herring (Arm) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Thierry Reding <[e-mail]>

Documentation/devicetree/bindings/firmware/nvidia,tegra186-bpmp.yaml

···
           - enum:
               - nvidia,tegra194-bpmp
               - nvidia,tegra234-bpmp
+              - nvidia,tegra264-bpmp
           - const: nvidia,tegra186-bpmp
       - const: nvidia,tegra186-bpmp