sandbox, test: add test for GPIO_HOG function

currently gpio hog function is not tested with "ut dm gpio"
so add some basic tests for gpio hog functionality.

For this enable GPIO_HOG in sandbox_defconfig, add
in DTS some gpio hog entries, and add testcase in
"ut dm gpio" command.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_flattree_defconfig b/configs/sandbox_flattree_defconfig
index 21f9047..05ce9bd 100644
--- a/configs/sandbox_flattree_defconfig
+++ b/configs/sandbox_flattree_defconfig
@@ -82,6 +82,7 @@
 CONFIG_DM_DEMO_SHAPE=y
 CONFIG_BOARD=y
 CONFIG_BOARD_SANDBOX=y
+CONFIG_GPIO_HOG=y
 CONFIG_PM8916_GPIO=y
 CONFIG_SANDBOX_GPIO=y
 CONFIG_I2C_CROS_EC_TUNNEL=y