mpu9250 allow a 2nd internal spi instance and remove px4fmu-v4 external

[dagar]

RE: #9372

Adding a fake "external" SPI bus to px4fmu-v4 isn't quite right because the rotations will likely be wrong (depending on the board).

This PR partially reverts #9372 and creates a second optional MPU device for the px4fmu-v4.

Note: this is still horrible, we need to find a better way to split the drivers and board configurations.

[AlexKlimaj]

Line 483 of "main.cpp" needs a "t".

while ((ch = getopt(argc, argv, "XISstR:")) != EOF) {

[dagar]

Thanks, updated.

[dagar]

@AlexKlimaj good to go?

[AlexKlimaj]

@dagar Yes, we have been flying it without issue. The only weird thing is the mag rotations in the driver that were needed to get it to match the lis3mdl.

[dagar]

Alright, let's merge this and continue discussing in another issue.

I still don't know how to rectify the mpu9250 mag rotations with the datasheet.