mtd: maps: physmap: Retain mtd-name property from dts

In physmap_flash_of_init() the maps[].name can be populated based on the
optional 'linux,mtd-name' property in the dts. Make sure this is
retained when filling in the rest of the map[] data.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200824025744.25992-1-chris.packham@alliedtelesis.co.nz