mtd: maps: physmap: Use platform_get_resource() to retrieve iomem resources

Stop manipulating the dev->resource array directly and use the
platform_get_resource() helper instead.

While at it, fix the loop check so that we never overflow the info->maps
and info->mtds array even if the number of resources attached to the
platform dev is higher than MAX_RESOURCES.

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>