[POWERPC] Convert axon_msi to an of_platform driver

Now that we create of_platform devices earlier on cell, we can make the
axon_msi driver an of_platform driver.  This makes the code cleaner in
several ways, and most importantly means we have a struct device.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>