Now that we can support the A31s pin controller with the A31 driver
using the new variants support, the independent A31s driver becomes
redundant.

Remove it.
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>