NVMe: high level model - review findings

[boekholt]

Comments gathered during review of nvme_top_i.sv:

• Handling of error conditions for NVMe read and write operations is missing.
• Is it possible to remove the #1 operation at the end of tasks nvme_cmd_read and `nvme_cmd_write?
• Should we remove the unused task ddr_axi_test?
• Is task axi_ddr_reset required?
• Set ACT_bresp and ACT_rresp to 2'hx together with releasing of ACT_bvalid and ACT_rvalid
• Add support for NVMe host admin registers
• Add NVMe lite model description (copy README.md from ibm github)
• Action Register Write Statemachine: Probably need to delay issuing of nvme_operation() in state WRITE_BUFFER similar to the WRITE_BURST implementation