xrp-20181022: XRP release 20181022:

- move all documentation to doc;
- drop status from xrp_*_{retain,release} functions;
- don't require sim-shmem node in the device tree for standalone ELF
  configurations;
- enable experimental support for cached shared memory on the DSP and
  expose xrp_device_enable_cache;
- introduce software-assisted edge IRQ mode for the simple hardware;
- add XEA3 support for the simple hardware;
- various bugfixes.