ASoC: imx-audmux: add RXFS/RXCLK defines for 6-wire connections

In asynchronous mode, a RxFS and RxClk connection needs to be made between
two ports. Add a define for the bit to be set in the *SEL fields.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
[m.felsch@pengutronix.de: fixed comment to include i.MX21 and 35]
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>