iavf: add support for indirect access to PHC time

Implement support for reading the PHC time indirectly via the
VIRTCHNL_OP_1588_PTP_GET_TIME operation.

Based on some simple tests with ftrace, the latency of the indirect
clock access appears to be about ~110 microseconds. This is due to the
cost of preparing a message to send over the virtchnl queue.

This is expected, due to the increased jitter caused by sending messages
over virtchnl. It is not easy to control the precise time that the
message is sent by the VF, or the time that the message is responded to
by the PF, or the time that the message sent from the PF is received by
the VF.

For sending the request, note that many PTP related operations will
require sending of VIRTCHNL messages. Instead of adding a separate AQ
flag and storage for each operation, setup a simple queue mechanism for
queuing up virtchnl messages.

Each message will be converted to a iavf_ptp_aq_cmd structure which ends
with a flexible array member. A single AQ flag is added for processing
messages from this queue. In principle this could be extended to handle
arbitrary virtchnl messages. For now it is kept to PTP-specific as the
need is primarily for handling PTP-related commands.

Use this to implement .gettimex64 using the indirect method via the
virtchnl command. The response from the PF is processed and stored into
the cached_phc_time. A wait queue is used to allow the PTP clock gettime
request to sleep until the message is sent from the PF.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Signed-off-by: Mateusz Polchlopek <mateusz.polchlopek@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

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