powerpc: handover page flags with a pgprot_t parameter

In order to avoid multiple conversions, handover directly a
pgprot_t to map_kernel_page() as already done for radix.

Do the same for __ioremap_caller() and __ioremap_at().

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>