drivers/fsi: Add sysfs files for FSI master & slave accesses

This change adds a 'raw' file for reads & writes, and a 'term' file for
the TERM command, and a 'break' file for issuing a BREAK.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Christopher Bostic <cbostic@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>