AP_Mount: Siyi, SToRM32_serial and ViewPro drivers always use 1st gimbal serial device

[rmackay9]

As discovered during this documentation update, the Siyi, SToRM32 and ViewPro drivers always try to use the first serial port that is set to Gimbal (e.g. SERIALx_PROTOCOL = 8). This means that if two serial gimbals are setup both will try and use the same serial port.

As a minimum we should change the call to the SerialManager to use the serial instance. This is sufficient for users only using serial gimbals.

This fix isn't a sufficient fix for the case where the 1st gimbal is not a serial gimbal (e.g. a PWM servo gimbal) because the 2nd gimbal (a serial gimbal) would try and use the 2nd serial port (e.g. the 2nd case of SERIALx_PROTOCOL = 8) when really it should use the first. To fix this we need to modify the serial gimbal consturctor to take in a serial instance argument similar to what we do for rangefinder.