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/picosam9g45_defconfig b/configs/picosam9g45_defconfig
index 0157f51..2921f2d 100644
--- a/configs/picosam9g45_defconfig
+++ b/configs/picosam9g45_defconfig
@@ -39,6 +39,7 @@
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_DM=y
 CONFIG_SPL_DM=y
+CONFIG_AT91_GPIO=y
 CONFIG_USB=y
 CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_STORAGE=y