mtd: core: always create master device

Create master device without partition when
CONFIG_MTD_PARTITIONED_MASTER flag is unset.

This streamlines device tree and allows to anchor
runtime power management on master device in all cases.

Signed-off-by: Alexander Usyskin <alexander.usyskin@intel.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>