SITL XPlane DRef Mapping for Axis 7, 8.

#### Issue details

With X-Plane 12 and DREF mapping for SITL XPlane, the simulation code only seems to be looking for joystick axis numbers 1-6; however, some joysticks axes such as roll are assigned as axis 7 and axis 8 by X-Plane. Is it possible add support for joysticks using axis 7 and 8? From Discord post from another user below:

"Update on my issues. First, I got the joystick to work with Mission Planner and X-Plane. X-Plane 12 maps your joystick(s) in an array sim/joystick/joystick_axis_values[500]. If it is not mapped to axis[1 to 6], it will never work in Mission Planner. I did 3 X-Plane 12 installations on 3 different computers and they all had different axis mapped for my single Logitech Extreme 3D Pro joystick. By looking at the Mission Planner X-Plane simulation code, it's looking for the raw axis values packets which only has axis[1 to 6] and 2 buttons. If your joystick is mapped to other axis, the raw axis values will be stuck at a fixed value at least that's what it did for me."

#### Version
MP 1.3.81

#### Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ x ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
SITL Plane

#### Hardware type
NA

#### Logs
NA

Start by locating the Mission Planner SITL XPlane input handling and its raw axis mapping, using the X-Plane 12 DREF and joystick_axis_values details in the issue. Verify how axes 1-6 are currently handled and determine what must change for axes 7 and 8; done means those mappings work for SITL Plane with the reported joystick setup.

Written by the indexing model from the issue text.