dt: add helper function to read u32 arrays

Rework of_property_read_u32 to read an array of values. Then
of_property_read_u32 becomes an inline with array size of 1.

Also make struct device_node ptr const.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>