Convert CONFIG_SPL_GPIO_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/gwventana_defconfig b/configs/gwventana_defconfig
index 0c245cd..2e68168 100644
--- a/configs/gwventana_defconfig
+++ b/configs/gwventana_defconfig
@@ -1,5 +1,6 @@
 CONFIG_ARM=y
 CONFIG_ARCH_MX6=y
+CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_TARGET_GW_VENTANA=y
 CONFIG_SPL_STACK_R_ADDR=0x18000000
 CONFIG_DM_SERIAL=y