cpufreq: OMAP: cleanup for multi-SoC support, move into drivers/cpufreq

Move OMAP cpufreq driver from arch/arm/mach-omap2 into
drivers/cpufreq, along with a few cleanups:

- generalize support for better handling of different SoCs in the OMAP
- use OPP layer instead of OMAP clock internals for frequency table init

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
[khilman@ti.com: move to drivers]
Signed-off-by: Kevin Hilman <khilman@ti.com>