Convert CONFIG_AT91_GPIO to Kconfig

This converts the following to Kconfig:
   CONFIG_AT91_GPIO

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/snapper9g20_defconfig b/configs/snapper9g20_defconfig
index 3ef091f..e1c5c2e 100644
--- a/configs/snapper9g20_defconfig
+++ b/configs/snapper9g20_defconfig
@@ -29,6 +29,7 @@
 CONFIG_CMD_FAT=y
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_AT91_GPIO=y
 CONFIG_CMD_PCA953X=y
 # CONFIG_MMC is not set
 CONFIG_MTD=y