powerpc: move find_linux_pte_or_hugepte and gup_hugepte to common code

We will use this in the later patch for handling THP pages

Reviewed-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>