mtd: maps: Merge gpio-addr-flash.c into physmap-core.c

Controlling some MSB address lines using GPIOs is just a small
deviation from the generic physmap logic, and merging those two drivers
allows us to share most of the probe logic, which is a good thing.

Also, the gpio-addr-flash driver is unused since the removal of
the blackfin arch in v4.17, so we can safely remove the old driver
without risking breaking existing boards.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Tested-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>