powerpc/ps3: Fix ioremap of spu shadow regs

Fix the ioremap of the spu shadow regs on the PS3.

The current PS3 hypervisor requires the spu shadow regs to be
mapped with the PTE page protection bits set as read-only (PP=3).
This implementation uses the low level __ioremap() to bypass the
page protection settings inforced by ioremap_flags() to get the
needed PTE bits set for the shadow regs.

This fixes a runtime failure on the PS3 introduced by the powerpc
ioremap_prot rework of commit a1f242ff
("powerpc ioremap_prot").

Signed-off-by: Masakazu Mokuno <mokuno@sm.sony.co.jp>
CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>