powerpc/cell: Rename ipi functions to match current abstractions

Rename functions and arguments to reflect current usage.  iic_cause_ipi
becomes iic_message_pass and iic_ipi_to_irq becomes iic_msg_to_irq,
and iic_request_ipi now takes a message (msg) instead of an ipi number.
Also mesg is renamed to msg.

Commit f1072939 (powerpc: Remove checks for MSG_ALL and
MSG_ALL_BUT_SELF) connected the smp_message_pass hook for cell to the
underlying iic_cause_IPI, a platform unique name.  Later 23d72bfd
(powerpc: Consolidate ipi message mux and demux) added a cause_ipi
hook to the smp_ops, also used in message passing, but for controllers
that can not send 4 unique messages and require multiplexing.  It is
even more confusing that the both take two arguments, but one is the
small message ordinal and the other is an opaque long data associated
with the cpu.

Since cell iic maps messages one to one to ipi irqs, rename the
function and argument to translate from ipi to message.  Also make it
clear that iic_request_ipi takes a message number as the argument
for which ipi to create and request.

No functionional change, just renames to avoid future confusion.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>