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/axm_defconfig b/configs/axm_defconfig
index 78e6c8d..1615d8a 100644
--- a/configs/axm_defconfig
+++ b/configs/axm_defconfig
@@ -58,6 +58,7 @@
 CONFIG_HAVE_BLOCK_DEVICE=y
 CONFIG_CLK=y
 CONFIG_CLK_AT91=y
+CONFIG_AT91_GPIO=y
 # CONFIG_MMC is not set
 CONFIG_MTD=y
 CONFIG_MTD_RAW_NAND=y