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/cm_t335_defconfig b/configs/cm_t335_defconfig
index 0f52879..1ba17b2 100644
--- a/configs/cm_t335_defconfig
+++ b/configs/cm_t335_defconfig
@@ -1,5 +1,6 @@
 CONFIG_ARM=y
 CONFIG_TARGET_CM_T335=y
+CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_EXT_SUPPORT=y
 CONFIG_SPL_FAT_SUPPORT=y
 CONFIG_SPL=y