libplatsupport: Add helper functions to FDT

These new helper functions in the FDT interface are mostly convenience
wrappers over the existing register and IRQ walking functions. Instead
of providing a callback, the user can present an offset and the helper
functions will attempt to map that particular register block or
interrupt.

libplatsupport: Use const for static strings

This is more compatible with existing interfaces.

trivial: Add docs to FDT and IRQ chip parsing libs

libplatsupport: Add FDT parsing library

This FDT parsing library is a collection of utility functions which
parses the FDT with the help of libfdt to simplify the common use cases
of the FDT.