[ARM] pgtable: rearrange file PTE bit allocation

For future compatibility, we need to ensure that swap and file Linux
PTEs conform with the hardware PTEs "fault" encoding.  Swap PTEs
already fit in with this, but file PTEs do not.  Shift them by one
bit to ensure that they conform, using bit 2 to distinguish between
swap and file PTEs.

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