CLKS signal for McBSP ports can be selected from internal (PRCM) or
external (ABE_CLKS pin) source.  To be able to use existing code we
need to create clock aliases consistent among OMAP2/3/4.

Based on a patch from Péter Ujfalusi <peter.ujfalusi@ti.com>;
the patch description above is his.
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Péter Ujfalusi <peter.ujfalusi@ti.com>
Cc: Tony Lindgren <tony@atomide.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>