bus: Add support for Tegra Generic Memory Interface

The Generic Memory Interface bus can be used to connect high-speed
devices such as NOR flash, FPGAs, DSPs...

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: Jon Hunter <jonathanh@nvidia.com>
[treding@nvidia.com: symmetry and coding style OCD]
Signed-off-by: Thierry Reding <treding@nvidia.com>