physmap_of sort of depends on the gemini and versatile modules (when
they're enabled), but this isn't expressed in Kconfig. Let's just merge
the modules all together, when enabled. Then we can avoid exporting a
few symbols, and the versatile and gemini code can now be modular again
(the below commit accidentally made them built-in only).

Resolves errors like this:

ERROR: "of_flash_probe_versatile" [drivers/mtd/maps/physmap_of.ko] undefined!
ERROR: "of_flash_probe_gemini" [drivers/mtd/maps/physmap_of.ko] undefined!

Fixes: 56ff337e

 ("mtd: physmap_of: add a hook for Gemini flash probing")
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>