Formalize, in kernel documentation, the patch acceptance policy for
arch/riscv. In summary, it states that as maintainers, we plan to
only accept patches for new modules or extensions that have been
frozen or ratified by the RISC-V Foundation.

We've been following these guidelines for the past few months. In the
meantime, we've received quite a bit of feedback that it would be
helpful to have these guidelines formally documented.

Based on a suggestion from Matthew Wilcox, we also add a link to this
file to Documentation/process/index.rst, to make this document easier
to find. The format of this document has also been changed to align
to the format outlined in the maintainer entry profiles, in accordance
with comments from Jon Corbet and Dan Williams.

Signed-off-by: Paul Walmsley
Reviewed-by: Palmer Dabbelt
Cc: Palmer Dabbelt
Cc: Albert Ou
Cc: Krste Asanovic
Cc: Andrew Waterman
Cc: Matthew Wilcox
Cc: Dan Williams
Cc: Jonathan Corbet