cpufreq: OMAP: fix clock usage to be SoC independent, remove plat/ includes

OMAP core code now has SoC-independent clock alias for the scalable
CPU clock.  Using it means driver is SoC independent and will work for
AM3xxx SoCs as well as OMAP1/3/4.

While here, remove some unnecessary plat/ includes that are
interfering with multi-subarch ARM kernels.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
[tony@atomide.com: updated already changed clock aliases]
Signed-off-by: Tony Lindgren <tony@atomide.com>
[khilman@ti.com: minor shortlog/changelog updates]
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Kevin Hilman <khilman@ti.com>