powerpc: Change the doorbell IPI calling convention

Change the doorbell callers to know about their msgsnd addressing,
rather than have them set a per-cpu target data tag at boot that gets
sent to the cause_ipi functions. The data is only used for doorbell IPI
functions, no other IPI types, so it makes sense to keep that detail
local to doorbell.

Have the platform code understand doorbell IPIs, rather than the
interrupt controller code understand them. Platform code can look at
capabilities it has available and decide which to use.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>