Documentation: RISC-V: Allow patches for non-standard behavior

The patch acceptance policy forbids accepting support for non-standard
behavior.  This policy was written in order to both steer implementers
towards the standards and to avoid coupling the upstream kernel too
tightly to vendor-specific features.  Those were good goals, but in
practice the policy just isn't working: every RISC-V system we have
needs vendor-specific behavior in the kernel and we end up taking that
support which violates the policy.  That's confusing for contributors,
which is the main reason we have a written policy in the first place.

So let's just start taking code for vendor-defined behavior.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Link: https://lore.kernel.org/all/alpine.DEB.2.21.999.2211181027590.4480@utopia.booyaka.com/
[Palmer: merge in Paul's suggestions]
Link: https://lore.kernel.org/r/20221207020815.16214-3-palmer@rivosinc.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>