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.

[[email protected]: Proposal and initial implementation]
Signed-off-by: Thomas Abraham <[email protected]>
[grant.likely: some word smithing and be more defensive validating the string]
Signed-off-by: Grant Likely <[email protected]>
2 files changed