serial: 8250_dw: Add support for IrDA SIR mode

Add a set_ldisc function to enable/disable IrDA SIR mode according to
the new line discipline, if IrDA SIR mode is supported by the hardware
configuration.

Signed-off-by: Ed Blake <ed.blake@imgtec.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>