[PATCH Linux] powerpc: add documentation for HWCAPs

[Florian Weimer]

* Nicholas Piggin:

> +2. Facilities
> +-------------
> +The Power ISA uses the term "facility" to describe a class of instructions,
> +registers, interrupts, etc. The presence or absence of a facility indicates
> +whether this class is available to be used, but the specifics depend on the
> +ISA version. For example, if the VSX facility is available, the VSX
> +instructions that can be used differ between the v3.0B and v3.1B ISA
> +verstions.

The 2.07 ISA manual also has categories.  ISA 3.0 made a lot of things
mandatory.  It may make sense to clarify that feature bits for mandatory
aspects of the ISA are still set, to help with backwards compatibility.

Thanks,
Florian