mtd: maps: Rename physmap_of_{versatile, gemini} into physmap-{versatile, gemini}

Now that the physmap_of driver is gone, the gemini and versative
extensions are part of the physmap driver. Rename the source files and
the config option to reflect this.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>