gpiolib: Get rid of ARCH_NR_GPIOS

Since commit 14e85c0e ("gpio: remove gpio_descs global array")
there is no limitation on the number of GPIOs that can be allocated
in the system since the allocation is fully dynamic.

ARCH_NR_GPIOS is today only used in order to provide downwards
gpiobase allocation from that value, while static allocation is
performed upwards from 0. However that has the disadvantage of
limiting the number of GPIOs that can be registered in the system.

To overcome this limitation without requiring each and every
platform to provide its 'best-guess' maximum number, rework the
allocation to allocate upwards, allowing approx 2 millions of
GPIOs.

In order to still allow static allocation for legacy drivers, define
GPIO_DYNAMIC_BASE with the value 512 as the start for dynamic
allocation. The 512 value is chosen because it is the end of
the current default range so all current static allocations are
expected to be below that value. Of course that's just a rough
estimate based on the default value, but assuming static
allocations come first, even if there are more static allocations
it should fit under the 512 value.

In the future, it is expected that all static allocations go away
and then dynamic allocation will be patched to start at 0.

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