OMAP2: clock: fix misc. OMAP24xx compile issues

- Convert use of non-existant cpu_is_omap2xxx() to cpu_is_omap24xx()
- Change prefix of OMAP2 CPUfreq init/exit functions from
  omap2xxx_ to omap2_ as used in clock.c
- fix 'possible use of uninitialized variable' warning in
  omap2xxx_clk_init()

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>