RISC-V Device Tree Documentation

[Palmer Dabbelt]

As part of submitting the RISC-V Linux port it was suggested that we write some
documentation that describes our device tree bindings.  Our port is still in
progress, but it was suggested that we start sending patches to the relevant
parties.  There is one outstanding issue to resolve with the PLIC bindings, but
I think the other two patches should be sane.