mtd: Make {MTDIDS, MTDPARTS}_DEFAULT visible when MTD_PARTITIONS is selected

gwventana configs are relying on CMD_UBI to select CMD_MTDPARTS,
which is then making {MTDIDS,MTDPARTS}_DEFAULT options available.

We are about to remove the 'select CMD_MTDPARTS' statement in the
CMD_UBI entry, but if we do that without first making sure
{MTDIDS,MTDPARTS}_DEFAULT are visible, we end up with a build
failure when building gwventana configs.

Address that by adding a depends on MTD_PARTITIONS to
{MTDIDS,MTDPARTS}_DEFAULT which does the trick since CMD_UBI selects
MTD_UBI which in turn selects MTD_PARTITIONS.

We also get rid of the depends on CMD_MTD, since CMD_MTD also selects
MTD_PARTITIONS.

Reported-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Jagan Teki <jagan@openedev.com>