target/armv8: regularly send keep_alive packet.

target/armv8: regularly send keep_alive packet.

Flushing all d-cache may be a long operation.
We need to send keep_alive regularly to keep the connection alive.
If not done a warning is emitted.

Change-Id: I52c3ee9a9f9b8a1dc0b8d5439e8b71212f56165a
Signed-off-by: Adrien Grassein <agrassein@nanoxplore.com>
Signed-off-by: Adrien Charruel <acharruel@nanoxplore.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8659
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins