mtd: Fix MTD_OF_PARTS for all arch and minor tidy of MTD_PARTITIONS

MTD_OF_PARTS should be possible on all architectures, not just
powerpc and microblaze, and it probably should not be a user
selectable option.  Neither does it need to be in a separate module.

Also, rework MTD Kconfig to group options dependant on MTD_PARTITIONS
into a if/endif block.  Do the same for MTD_REDBOOT_PARTS.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>