mtd: add mtd_ooblayout_xxx() helper functions

mtd: add mtd_ooblayout_xxx() helper functions

In order to make the ecclayout definition completely dynamic we need to
rework the way the OOB layout are defined and iterated.

Create a few mtd_ooblayout_xxx() helpers to ease OOB bytes manipulation
and hide ecclayout internals to their users.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
[Linux commit: 75eb2cec251fda33c9bb716ecc372819abb9278a]
[masahiro:
 cherry-pick more code from adbbc3bc827eb1f43a932d783f09ba55c8ec8379]
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>