mtd: add new API for handling MTD registration

Lots (nearly all) mtd drivers contain nearly the similar code that
calls parse_mtd_partitions, provides some platform-default values, if
parsing fails, and registers  mtd device.

This is an aim to provide single implementation of this scenario:
mtd_device_parse_register() which will handle all this parsing and
defaults.

Artem: amended comments

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>