cxl/mbox: Move command definitions to common location

In preparation for cxl_test to mock responses to mailbox command
requests, move some definitions from core/mbox.c to cxlmem.h.

No functional changes intended.

Acked-by: Ben Widawsky <ben.widawsky@intel.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/163116439547.2460985.10457111177103589574.stgit@dwillia2-desk3.amr.corp.intel.com


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