ARM: OMAP: mcbsp: Start generalize signal muxing functions

This generalizes the omap2_mcbsp1_mux_clkr_src and omap2_mcbsp1_mux_fsr_src
implementation between generic McBSP and OMAP2 specific McBSP code. These
functions are used to select source for CLKR and FSR signals on OMAP2+.

Start generalizing the code by implementing an optional mux_signal function
pointer in platform data that will implement the actual muxing and which is
called now from omap2_mcbsp1_mux_clkr_src and omap2_mcbsp1_mux_fsr_src.
These functions are to be removed later and cleanup the API so that
mux_signal gets its arguments directly from client code.

Signed-off-by: Jarkko Nikula <jarkko.nikula@bitmer.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Tested-by: Janusz Krzysztofik <jkrzyszt@tis.icnet.pl>
Signed-off-by: Tony Lindgren <tony@atomide.com>