powerpc/ps3: replace __ioremap() by ioremap_prot()

__ioremap() is similar to ioremap_prot() except that ioremap_prot()
does a few sanity changes in addition.

The flags used by PS3 are not impacted by those changes so for
PS3 both functions are equivalent.

At the same time, drop parts of the comment that have been invalid
since commit e58e87ad ("powerpc/mm: Update _PAGE_KERNEL_RO")

Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/36bff5d875ff562889c5e12dab63e5d7c5d1fbd8.1566309262.git.christophe.leroy@c-s.fr