mtd: physmap-core: Restore map_rom fallback

mtd: physmap-core: Restore map_rom fallback

When the exact mapping type driver was not available, the old
physmap_of_core driver fell back to mapping the region as ROM.
Unfortunately this feature was lost when the DT and pdata cases were
merged.  Revive this useful feature.

Fixes: 642b1e8dbed7bbbf ("mtd: maps: Merge physmap_of.c into physmap-core.c")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/550e8c8c1da4c4baeb3d71ff79b14a18d4194f9e.1693407371.git.geert+renesas@glider.be