dt: add empty of_alias_get_id() for non-dt builds

Add function of_alias_get_id() reporting -ENOSYS
for non-dt builds, so that drivers migrating to dt
can save some '#ifdef CONFIG_OF'.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>