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: NConor Dooley <conor.dooley@microchip.com>
Reviewed-by: NAnup Patel <anup@brainfault.org>
Signed-off-by: NPaul 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.comSigned-off-by: NPalmer Dabbelt <palmer@rivosinc.com>

Documentation/riscv/patch-acceptance.rst

@@ -29,7 +29,11 @@ their own custom extensions.  These custom extensions aren't required
 to go through any review or ratification process by the RISC-V
 Foundation.  To avoid the maintenance complexity and potential
 performance impact of adding kernel code for implementor-specific
-RISC-V extensions, we'll only accept patches for extensions that
-have been officially frozen or ratified by the RISC-V Foundation.
-(Implementors, may, of course, maintain their own Linux kernel trees
-containing code for any custom extensions that they wish.)
+RISC-V extensions, we'll only consider patches for extensions that either:
+
+- Have been officially frozen or ratified by the RISC-V Foundation, or
+- Have been implemented in hardware that is widely available, per standard
+  Linux practice.
+
+(Implementors, may, of course, maintain their own Linux kernel trees containing
+code for any custom extensions that they wish.)