Merge branch 'for-6.15/features' into fwctl

Add CXL mailbox Features commands enabling. This is also preparation for
CXL fwctl enabling. The same code will also be utilized by the CXL EDAC
enabling. The commands 'Get Supported Features', 'Get Feature', and 'Set
Feature' are enabled for kernel usages.

Required for the CXL fwctl driver.

* branch 'for-6.15/features'
  cxl: Setup exclusive CXL features that are reserved for the kernel
  cxl/mbox: Add SET_FEATURE mailbox command
  cxl/mbox: Add GET_FEATURE mailbox command
  cxl/test: Add Get Supported Features mailbox command support
  cxl: Add Get Supported Features command for kernel usage
  cxl: Enumerate feature commands
  cxl: Refactor user ioctl command path from mds to mailbox

Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>