[FREELDR] Limit the usage of DiskStopFloppyMotor() in hardware/platform-specific...

[FREELDR] Limit the usage of DiskStopFloppyMotor() in hardware/platform-specific code.

- Move DiskStopFloppyMotor() calls into the implementations of
  Boot(New)LinuxKernel() and Reboot() HW functions, and the explanation
  comments in ChainLoadBiosBootSectorCode().

- Remove unneeded DiskStopFloppyMotor() dummies in ARM and PPC code.

- Use more adequate bitmask value to be sent to floppy's Digital Output
  Register for shutting down its motor (based on OSDev & our floppy
  controller driver).