add BTT SKR-2 pins

[bigtreetech]

• SKR-2 motherboard added a function of stepper driver anti reverse plug protect. It needed to supply power to the stepper driver by control signal wire(PC13 in SKR-2). RRF does not have this function at present, so the driver cannot supply power. But there is now a compromise to make it work. We can set PC13 to atx.powerPin. In this way, the signal wire will control the power supply to the driver before TMC driver initialization.

add

atx.powerPin = C.13; // safe power
atx.powerPinInverted = false

in board.txt But in this way, it is impossible to use M80 / M81 to control the power supply of the whole printer.

[gloomyandy]

I've merged the pins and support for both rotation detection and the SKR 2 driver power control into V3.3beta1_6, so I'm closing this pull request.