uboot-imx

dm: test: Add GPIO open drain tests

Add some tests for the new open drain setting feature of the GPIO
uclass, and extend the capabilities of the sandbox GPIO driver
accordingly.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: York Sun <york.sun@nxp.com>