This converts the following to Kconfig:
   CONFIG_MXS_GPIO

Travis-CI: https://travis-ci.org/lmajewski/u-boot-dfu/builds/571260789Signed-off-by: NLukasz Majewski <lukma@denx.de>
Acked-by: NPeng Fan <peng.fan@nxp.com>
Acked-by: NJagan Teki <jagan@amarulasolutions.com>