test: dm: pinmux: Get LED2 udevice in the pinmux test

The UT reinitializes the pin controller state, get LED2 udevice
to trigger its probe and configure the pin controller pin state
as it is expected by the test.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Cc: Sean Anderson <seanga2@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Steven Lawrance <steven.lawrance@softathome.com>