ARM: tegra: document use of standard DMA DT bindings

Update all the Tegra DT bindings to require the standard dmas/dma-names
properties rather than non-standard nvidia,dma-request-selector property.

This is a DT-ABI-incompatible change. It is the second of two changes
required for me to consider the Tegra DT bindings as stable, the other
being the previous conversion to the common reset bindings.

Signed-off-by: Stephen Warren <swarren@nvidia.com>