[POWERPC] Update axon_msi to use dcr_host_t.base

Now that dcr_host_t contains the base address, we can use that in the
axon_msi code, rather than storing it separately.

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>