ARM: OMAP: mcbsp: Start generalize omap2_mcbsp_set_clks_src

This generalizes the omap2_mcbsp_set_clks_src implementation between generic
McBSP and OMAP2 specific McBSP code. Currently this function is used to
select either internal fclk or clks pin as a McBSP CLKS source on OMAP2+.

Implement generalization by having an optional set_clk_src function pointer
in platform data that is used to select parent for a given clock. Idea is to
pass higher level source clock name (later coming from client driver) that
platform specific code will map to platform specific clock name.

API cleanup between McBSP and client code comes later.

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>