gpio: fix test.py for gpio label lookup

commit 2bd261dd1712 ("gpio: search for gpio label if gpio is not found through bank name")

disabled DM_GPIO_LOOKUP_LABEL which is needed
in sandbox defconfigs, as we have tests for this
functionality.

Signed-off-by: Heiko Schocher <hs@denx.de>
diff --git a/configs/sandbox_flattree_defconfig b/configs/sandbox_flattree_defconfig
index 9fe4c9d..dfcc435 100644
--- a/configs/sandbox_flattree_defconfig
+++ b/configs/sandbox_flattree_defconfig
@@ -84,6 +84,7 @@
 CONFIG_BOARD=y
 CONFIG_BOARD_SANDBOX=y
 CONFIG_GPIO_HOG=y
+CONFIG_DM_GPIO_LOOKUP_LABEL=y
 CONFIG_PM8916_GPIO=y
 CONFIG_SANDBOX_GPIO=y
 CONFIG_I2C_CROS_EC_TUNNEL=y