of/selftest: Use selftest() macro throughout

Some of the selftests are open-coded. Others use the selftest() macro
defined in drivers/of/selftest.c. The macro makes for cleaner selftest
code, so refactor the of_parse_phandle_with_args() tests to use it.

Cc: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>