[POWERPC] cell: Generalize io-workarounds code

This splits cell io-workaround code into spider-pci dependent code and
a generic part, and also moves io-workarounds initialization into
cell_setup_phb.

Signed-off-by: Kou Ishizaki <kou.ishizaki@toshiba.co.jp>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>