cxl/mbox: Add background cmd handling machinery

This adds support for handling background operations, as defined in
the CXL 3.0 spec. Commands that can take too long (over ~2 seconds)
can run in the background asynchronously (to the hardware).

The driver will deal with such commands synchronously, blocking all
other incoming commands for a specified period of time, allowing
time-slicing the command such that the caller can send incremental
requests to avoid monopolizing the driver/device. Any out of sync
(timeout) between the driver and hardware is just disregarded as
an invalid state until the next successful submission. Such timeouts
are considered a rare occurrence, either a real device problem or a
driver issue that needs to reduce the size of the background operation
to fit the timeout.

On devices where mbox interrupts are supported, this will still use
a poller that will wakeup in the specified wait intervals. The irq
handler will simply awake the blocked cmd, which is also safe vs a
task that is either waking (timing out) or already awoken. Similarly
any irq setup error during the probing falls back to polling, thus
avoids unnecessarily erroring out.

Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Link: https://lore.kernel.org/r/20230523170927.20685-5-dave@stgolabs.net
Signed-off-by: Dan Williams <dan.j.williams@intel.com>