tty: serial: 8250_dw: Implement suspend/resume

Implement suspend and resume callbacks for DesignWare 8250 driver.
They're simple wrappers around serial8250_{suspend,resume}_port.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>