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/am3517_crane_defconfig b/configs/am3517_crane_defconfig
index a6a83fc..329f1f1 100644
--- a/configs/am3517_crane_defconfig
+++ b/configs/am3517_crane_defconfig
@@ -4,6 +4,7 @@
 CONFIG_BOOTDELAY=10
 CONFIG_SPL=y
 # CONFIG_SPL_EXT_SUPPORT is not set
+# CONFIG_SPL_GPIO_SUPPORT is not set
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="AM3517_CRANE # "
 # CONFIG_CMD_IMI is not set