dtc: Add some documentation for the dts formta

[Segher Boessenkool]

> This patch adds a dts-format.txt in the Documentation directory, with
> an introduction to the dtc source format.

Looks good, some minor nits...

> +* A 64-bit value is represented with two 32-bit cells.
> +
> +	e.g. clock-frequency = <0x00000001 0x00000000>;

Please add that this is big-endian order always.

> +* Values may have several comma-separated components, which are
> +  concatenated together.
> +	e.g. compatible = "ns16550", "ns8250";

You might want to clarify that the terminating 0 of ns16550 becomes
part of the property, i.e. this is not like strcat() in C.


Segher