OMAP2420: clock: add MCBSP_CLKS node and clkdev aliases

Add the MCBSP_CLKS clock and the clksel structures needed to support clock
framework-based source switching for McBSP 1 and 2.  Also, add clkdev
aliases on the parent clocks for the McBSP source switching code, added
in a subsequent patch.

Signed-off-by: Paul Walmsley <paul@pwsan.com>