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/gurnard_defconfig b/configs/gurnard_defconfig
index 0d1a463..a679c0f 100644
--- a/configs/gurnard_defconfig
+++ b/configs/gurnard_defconfig
@@ -32,6 +32,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="at91sam9g45-gurnard"
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_AT91_GPIO=y
 CONFIG_MTD=y
 CONFIG_MTD_RAW_NAND=y
 # CONFIG_SYS_NAND_USE_FLASH_BBT is not set