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>