davinci: DA850/OMAP-L138: add frequency scaling support

Adds basic frequency scaling support for DA850/OMAP-L138.

Currently, frequency scaling only on PLL0 is supported. No scaling of PLL1
as yet.

Peripherals like MMC/SD which have a clock input synchronous with
ARM clock will not work well since the clock will change behind their backs.
Support for notification to such devices to adjust themselves to the
new frequency will be added in later patches. Current defconfigs keep
CPUFreq disabled so it will not affect normal operation.

The OPP defintions assume clock input of 24MHz to the SoC. This is inline
with hardcoding of input frequency in the <soc>.c files. At some point
this will need to move into board dependent code as new boards appear with
a different reference clock.

Tested on OMAP-L138 EVM with ondemand governer and a shell script to
vary processor load.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>