ARM: ux500: select the DB8540 pin controller

The pin controller is an inherent part of the SoC, without
it the system will not boot, thus it needs to be selected
from Kconfig.

Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>