test: pinmux: add pincontrol-gpio for pin configuration

test: pinmux: add pincontrol-gpio for pin configuration

Add a simple pincontrol associated to the sandbox gpio driver,
that allows to check pin configuration with the command pinmux.

The pinmux test is also updated to test behavior with 2 pincontrols.

Example to check LED pin configuration:

=> pinmux list
| Device                        | Driver                        | Parent
| pinctrl-gpio                  | sandbox_pinctrl_gpio          | root_driver
| pinctrl                       | sandbox_pinctrl               | root_driver

=> pinmux dev pinctrl-gpio

=> pinmux status

a0        : gpio input .
a1        : gpio input .
a2        : gpio input .
a3        : gpio input .
a4        : gpio input .
a5        : gpio output .
a6        : gpio output .
...

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>