test: pinmux: Add test for pin muxing

This extends the pinctrl-sandbox driver to support pin muxing, and adds a
test for that behaviour. The test is done in C and not python (like the
existing tests for the pinctrl uclass) because it needs to call
pinctrl_select_state.  Another option could be to add a command that
invokes pinctrl_select_state and then test everything in
test/py/tests/test_pinmux.py.

The pinctrl-sandbox driver now mimics the way that many pinmux devices
work.  There are two groups of pins which are muxed together, as well as
four pins which are muxed individually. I have tried to test all normal
paths. However, very few error cases are explicitly checked for.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>