tools/testing/cxl: add firmware update emulation to CXL memdevs

Add emulation for the 'Get FW Info', 'Transfer FW', and 'Activate FW'
CXL mailbox commands to the cxl_test emulated memdevs to enable
end-to-end unit testing of a firmware update flow. For now, only
advertise an 'offline activation' capability as that is all the CXL
memdev driver currently implements.

Add some canned values for the serial number fields, and create a
platform device sysfs knob to calculate the sha256sum of the firmware
image that was received, so a unit test can compare it with the original
file that was uploaded.

Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
Cc: Russ Weight <russell.h.weight@intel.com>
Cc: Alison Schofield <alison.schofield@intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Ben Widawsky <bwidawsk@kernel.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Link: https://lore.kernel.org/r/20230602-vv-fw_update-v4-4-c6265bd7343b@intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>