[lib][hwreg] Add a C++ library for accessing MMIO registers

This provides a mechanism for defining register layouts and handles
accessing them with the correct access-width, dealing with reserved
bits, and extracting fields.

The library is derived from an earlier framework written by Mark Seaborn:
https://fuchsia.googlesource.com/garnet/+/ced4162ec279c79e7ce33f39bdd5e3edd2f8aaaa/drivers/gpu/msd-intel-gen/src/register_bitfields.h

ZX-693 # comment support lib for driver

Change-Id: I13e08e6002418ccd1ec8fcee10873dfdf23c3e7f