of: fix of_update_property()

The of_update_property() is intented to update a property in a node
and if the property does not exist, will add it.

The second search of the property is possibly won't be found, that
maybe removed by other thread just before the second search begain.

Using the __of_find_property() and __of_add_property() instead and
move them into lock operations.

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>