dt: add helper functions to read u32 and string property values

Add helper functions to retrieve unsigned integer and string property
values from properties of a device node. These helper functions can be
used to lookup a property in a device node, perform error checking and
read the property value.

[grant.likely@secretlab.ca: Proposal and initial implementation]
Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
[grant.likely: some word smithing and be more defensive validating the string]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>