asm-generic: qspinlock: Indicate the use of mixed-size atomics

The qspinlock implementation depends on having well behaved mixed-size
atomics.  This is true on the more widely-used platforms, but these
requirements are somewhat subtle and may not be satisfied by all the
platforms that qspinlock is used on.

Document these requirements, so ports that use qspinlock can more easily
determine if they meet these requirements.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Waiman Long <longman@redhat.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>