OMAP2/3/4 clock: combine all omap2_clk_functions

The struct clk_functions for OMAP2, 3, and 4 are all essentially the
same, so combine them.  This removes one multi-OMAP kernel impediment
and saves memory on multi-OMAP builds.

The stubs for omap2_clk_{init,exit}_cpufreq() code will removed once
the OPP layer code that's currently in Kevin's PM branch is merged.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>