dt-bindings: memory: tegra: Document DBB clock for Tegra264

dt-bindings: memory: tegra: Document DBB clock for Tegra264

Accesses to external memory are routed through the data backbone (DBB)
on Tegra264. A separate clock feeds this path and needs to be enabled
whenever an IP block makes an access to external memory. The external
memory controller driver is the best place to control this clock since
it knows how many devices are actively accessing memory.

Document the presence of this clock on Tegra264 only.

Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>