dm: core: Provide ofnode_find_subnode_unit()

dm: core: Provide ofnode_find_subnode_unit()

The ofnode_find_subnode() function currently processes things two
different ways, so the treatment of unit addresses differs depending on
whether OF_LIVE is enabled or not.

Add a new version which uses the ofnode API and add a test to check that
unit addresses can be matched correctly. Leave the old function in place
for the !OF_LIVE case, to avoid a code-size increase, e.g. on
firefly-rk3288

Signed-off-by: Simon Glass <sjg@chromium.org>