cpumask: alpha: Introduce cpumask_of_{node,pcibus} to replace {node,pcibus}_to_cpumask

Impact: New APIs

The old node_to_cpumask/node_to_pcibus returned a cpumask_t: these
return a pointer to a struct cpumask.  Part of removing cpumasks from
the stack.

I'm not sure the existing code even compiles, but new version is
straightforward.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Richard Henderson <rth@twiddle.net>