mtd: maps: physmap: Remove the MAX_RESOURCES limitation

Remove the MAX_RESOURCES limitation by dynamically allocating the
->mtds[] and ->maps[] at probe time based on the number of iomem
resources attached to the platform device.

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>