drivers: espi: xec: Add support for host-initiated traffic

Keep OOB Rx channel and interrupt always enabled.

Send callback when packet is received in OOB Rx channel
if asynchronous host-initiated handling is enabled.

Note that driver doesn't perform any buffering from packets,
so access to OOB Rx channel is gated by client's driver
packet retrieval.

Signed-off-by: Jose Alberto Meza <jose.a.meza.arellano@intel.com>