powerpc: rename iommu_num_pages function to iommu_nr_pages

This is a preparation patch for introducing a generic iommu_num_pages function.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Muli Ben-Yehuda <muli@il.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>