mtd: maps: Prepare merging of physmap and physmap_of

We want to merge the physmap and physmap_of driver, but before we can
do that we must prepare things to create physmap.o out of several .c
files. Rename physmap.c into physmap-core.c and add a new Makefile
rule to create physmap.o (right now it only contains physmap-core.o).

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>