OMAP2/3 clock: remove unnecessary includes and clean up header

Now that almost all of the code has been removed from clock2xxx.c and
clock34xx.c, many of the includes are now unnecessary and can be removed.
While we're here, standardize the initial comment blocks.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Richard Woodruff <r-woodruff2@ti.com>
Cc: Jouni Högander <jouni.hogander@nokia.com>