ARM: OMAP2: Move McBSP fck clock alias to hwmod data for OMAP2420

Remove the existing alias for pad_fck, prcm_fck from the clock data and
add them as opt_clks to the hwmod data.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>