mtd: phram: Make phram 64-bit compatible

phram was 32-bit limited by design. Machines are growing up, but phram
module is still useful. Update it. The patch is bigger than minimum,
because simple_strtoul() is obsolete.

Tested on MIPS64 and compile-tested for PPC (32 bit).

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nsn.com>
Reviewed-by: Joern Engel <joern@logfs.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>