OMAP3xxx: clock: add clkdev aliases for McBSP fclk source switching

The OMAP3 clock tree already contains the infrastructure to support
clock framework-based McBSP functional clock source switching.  But it
did not contain the clkdev aliases for the McBSP code to refer to the
parent clocks in an SoC integration-neutral way.  So, add the clkdev
aliases for the parent clocks.

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