powerpc: Implement and use pgprot_nx()

ioremap_page_range() calls pgprot_nx() vmap() and vmap_pfn()
clear execute permission by calling pgprot_nx().

When pgprot_nx() is not defined it falls back to a nop.

Implement it for powerpc then use it in early_ioremap_range().

Then the call to pte_exprotect() can be removed from ioremap_prot().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/5993a7a097e989af1c97fc4a6c011fefc67dbe6e.1695659959.git.christophe.leroy@csgroup.eu