dt/bindings: Add bindings for Tegra GMI controller

Document the devicetree bindings for the Generic Memory Interface (GMI)
bus driver found on Tegra SOCs.

Signed-off-by: Mirza Krak <mirza.krak@gmail.com>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Tested-on: Colibri T20/T30 on EvalBoard V3.x and GMI-Memory Board
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>