pmdomain: Add pm_test_nopop_bus and a driver for sync_state tests

Populate device-nodes under /pm_tests_nopop and hook up a bus/driver to
them.

Use "probe-delay-ms" to delay probe and/or use "probe-defer" to always
return -EPROBE_DEFER from their ->probe().

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>