OFW: Add helper functions for parsing xref based lists.
By using this functions, we can parse a list of tuples, each of them holds
xref and variable number of values.
This kind of list is used in DT for clocks, gpios, resets ...

Discussed with: ian, nwhitehorn
Approved by: kib (mentor)
Differential Revision: https://reviews.freebsd.org/D4316