Add the external memory controller as a child device of the memory
controller on Tegra186. The memory controller really represents the
memory subsystem that encompasses both the memory and external memory
controllers. The external memory controller uses the BPMP to obtain the
list of supported EMC frequencies and set the EMC frequency.

Also set up the dma-ranges property to describe that all memory clients
can address up to 40 bits using the memory controller client interface
(MCCIF), unless otherwise limited by the DMA engines of the hardware.
Signed-off-by: Thierry Reding <treding@nvidia.com>