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/taurus_defconfig b/configs/taurus_defconfig
index 554581b..67a6ade 100644
--- a/configs/taurus_defconfig
+++ b/configs/taurus_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_ARCH_AT91=y
 CONFIG_TARGET_TAURUS=y
+CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_DEFAULT_DEVICE_TREE="at91sam9g20-taurus"
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9G20,MACH_TYPE=2067,BOARD_TAURUS"
 CONFIG_BOOTDELAY=3