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/at91sam9x5ek_nandflash_defconfig b/configs/at91sam9x5ek_nandflash_defconfig
index 4a21a62..3521f15 100644
--- a/configs/at91sam9x5ek_nandflash_defconfig
+++ b/configs/at91sam9x5ek_nandflash_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_ARCH_AT91=y
 CONFIG_TARGET_AT91SAM9X5EK=y
+CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9X5,SYS_USE_NANDFLASH"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y