kconfig: Convert CONFIG_MXS_GPIO to Kconfig

This converts the following to Kconfig:
   CONFIG_MXS_GPIO

Travis-CI: https://travis-ci.org/lmajewski/u-boot-dfu/builds/571260789

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Acked-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>
diff --git a/configs/mx28evk_defconfig b/configs/mx28evk_defconfig
index 4cee901..7f000c8 100644
--- a/configs/mx28evk_defconfig
+++ b/configs/mx28evk_defconfig
@@ -38,6 +38,7 @@
 CONFIG_MTDPARTS_DEFAULT="mtdparts=gpmi-nand:3m(bootloader)ro,512k(environment),512k(redundant-environment),4m(kernel),512k(fdt),8m(ramdisk),-(filesystem)"
 CONFIG_CMD_UBI=y
 CONFIG_ENV_IS_IN_MMC=y
+CONFIG_MXS_GPIO=y
 CONFIG_MMC_MXS=y
 CONFIG_NAND=y
 CONFIG_NAND_MXS=y