OMAP clock: remove incorrect EXPORT_SYMBOL()s

The only symbols that should be exported are symbols that are to be
called from loadable kernel modules, e.g., device drivers.  In the
context of plat-omap/clock.c, these should only be the Linux clock
interface symbols as defined by include/linux/clk.h.  Core code
doesn't need these symbols to be exported.  Also, clean up an old
comment while here.

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