gpio: aggregator: Stop using ARCH_NR_GPIOS

ARCH_NR_GPIOS is used locally in aggr_parse() as the maximum number
of GPIOs to be aggregated together by the driver since
commit ec75039d

 ("gpio: aggregator: Use bitmap_parselist() for
parsing GPIO offsets").

Don't rely on the total possible number of GPIOs in the system but
define a local arbitrary macro for that, set to 512 which should be
large enough as it is also the default value for ARCH_NR_GPIOS.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>