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/etamin_defconfig b/configs/etamin_defconfig
index fd864ae..9d0470c 100644
--- a/configs/etamin_defconfig
+++ b/configs/etamin_defconfig
@@ -1,5 +1,6 @@
 CONFIG_ARM=y
 CONFIG_TARGET_ETAMIN=y
+CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SYS_MALLOC_F_LEN=0x2000
 CONFIG_SPL_FAT_SUPPORT=y
 CONFIG_DEFAULT_DEVICE_TREE="am335x-draco"