mtd: maps: physmap: Rename ->map and ->mtd into ->maps and ->mtds

The ->map and ->mtd fields are actually arrays of map and mtd objects.
Rename those fields into ->maps and ->mtds to make it obvious.

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>