cpufreq: OMAP: use get_cpu_device() instead of omap_device API

OMAP PM core code has moved to using the existing, generic CPU devices
for attaching OPPs, so the CPUfreq driver can now use the generic
get_cpu_device() API instead of the OMAP-specific omap_device API.

This allows us to remove the last <plat/*> include from this driver.

Cc: Paul Walmsley <paul@pwsan.com>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Kevin Hilman <khilman@ti.com>