mtd: physmap_of: really fix the physmap add-ons

The current way of building the of_physmap add-ons result in just
the add-on being in the object code, and not the actual core
implementation and regress the Gemini and Versatile.

Bake the physmap_of.o object by baking physmap_of_core.o and
adding the Versatile and/or Gemini add-ons to the final object.
Rename the source file physmap_of_core.c to get the desired
build components.

Suggested-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Fixes: 4f04f68e ("mtd: physmap_of: fixup gemini/versatile dependencies")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>