drm/xe/guc: Introduce Relay Communication for SR-IOV

There are scenarios where SR-IOV Virtual Function (VF) driver will
need to get additional data that is not available over VF MMIO BAR
nor could be queried from the GuC firmware and must be obtained
from the Physical Function (PF) driver.

To allow such communication between VF and PF drivers, GuC supports
set of H2G and G2H actions which allows relaying embedded messages,
that are otherwise opaque for the GuC.

To allow use of this communication mechanism, provide functions for
sending requests and handling replies and placeholder where we will
put handlers for incoming requests.

Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://lore.kernel.org/r/20240104222031.277-8-michal.wajdeczko@intel.com
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>