dm: core: Rename ofnode_get_chosen_prop()

This function is actually intended to read a string rather than a
property. All of its current callers use it that way. Also there is no way
to return the length of the property from this function.

Rename it to better indicate its purpose, using ofnode_read as the prefix
since this matches most other functions.

Also add some tests which are missing for these functions.

Signed-off-by: Simon Glass <sjg@chromium.org>