[ARM] Convert pmd_page() to be highmem safe

In the long run, we may want to place page tables in highmem.  However,
pmd_page() has traditionally been coded to convert the physical address
to a virtual one, which won't work with highmem pages.  Instead,
translate the physical address to a PFN, and then convert the PFN to a
struct page instead.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>