[MTD] mtdpart: Make all partition parsers return allocated array

Currently redboot and afx parser return allocated mtd_partition array
and cmdlinepart and ar7 return persistent array.

This patch make cmdlinepart and ar7 also return allocated array, so
that all users can free it regardless of parser type.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>