test: pinmux: add test for 'pinctrl-single' driver

The test adds two pinmux nodes to the device tree, one to test when a
register changes only one pin's mux (pinctrl-single,pins), and the other
to test when more than one pin's mux is changed (pinctrl-single,bits).
This required replacing the controller's register access functions when
the driver is used on sandbox.

Signed-off-by: Dario Binacchi <dariobin@libero.it>
Reviewed-by: Simon Glass <sjg@chromium.org>