fsi: Add fsi-master class

This change adds a device class for FSI masters, allowing access under
/sys/class/fsi-master/, and easier udev rules.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Acked-by: Alistair Popple <alistair@popple.id.au>
Link: https://lore.kernel.org/r/20191108051945.7109-2-joel@jms.id.au


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>