of: base: use of_property_read_string()

of_alias_scan() can use of_property_read_string() -- using the modern DT API
adds some prop sanity checks as a bonus; it does add couple LoCs but only
because the original code violated the 80-column limit...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Rob Herring <robh@kernel.org>