dt: add of_alias_scan and of_alias_get_id

The patch adds function of_alias_scan to populate a global lookup
table with the properties of 'aliases' node and function
of_alias_get_id for drivers to find alias id from the lookup table.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
[grant.likely: add locking and rework parse loop]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>