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/arch/arm/cpu/armv7/am33xx/Kconfig b/arch/arm/cpu/armv7/am33xx/Kconfig
index efbc219..54474c2 100644
--- a/arch/arm/cpu/armv7/am33xx/Kconfig
+++ b/arch/arm/cpu/armv7/am33xx/Kconfig
@@ -3,6 +3,9 @@
 config SPL_EXT_SUPPORT
 	default y
 
+config SPL_GPIO_SUPPORT
+	default y
+
 config TARGET_AM43XX_EVM
 	bool "Support am43xx_evm"
 	select TI_I2C_BOARD_DETECT