x86/microcode: Move away from using a fake platform device

Downloading firmware needs a device to hang off of, and so a platform device
seemed like the simplest way to do this.  Now that we have a faux device
interface, use that instead as this "microcode device" is not anything
resembling a platform device at all.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
Link: https://lore.kernel.org/2025070121-omission-small-9308@gregkh