mtd: constify mtd_partition

mtd_partition are not supposed to change at runtime.
Functions 'mtd_device_parse_register' working with const mtd_partition
provided by <linux/mtd/mtd.h>. So mark the non-const structs as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>