of: add __of_add_property() without lock operations

There two places will use the same code for adding one new property to
the DT node. Adding __of_add_property() and prepare for fixing
of_update_property()'s bug.

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>