[ARM] mem_init() cleanups

Make free_area() arguments pfn based, and return number of freed pages.
This will simplify highmem initialization later.

Also, codepages, datapages and initpages are actually codesize, datasize
and initsize.

Signed-off-by: Nicolas Pitre <nico@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>