powerpc: call_do_[soft]irq() takes a pointer to the stack

The purpose of the pointer given to call_do_softirq() and
call_do_irq() is to point the new stack. Currently that's the same
thing as the thread_info, but won't be with THREAD_INFO_IN_TASK.

So change the parameter to void* and rename it 'sp'.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Split out of larger patch]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>