mtd: maps: physmap: Leave assigned complex mappings

SoC-specific drivers might provide their own map->xxx()
implementations, and calling simple_map_init() unconditionally will
override those implementations.

Make sure map->read is NULL before calling simple_map_init().

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>