ARM: multiplatform: Sort the max gpio numbers.

When building a multiplatform kernel, we could end up with a smaller
number of GPIOs than the one required by the platform the kernel was
running on.

Sort the max GPIO number by descending order so that we always take the
highest number required.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>