hardware_gen: Add support for /reserved-memory node

Parse the /reserved-memory node per the Linux spec[1].
Ignore regions marked as 'no-map', but keep other regions
as they should be OK to use as RAM.

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt?h=v5.0-rc6