omap: gpu: Addd sysfs interface for frequency_limit

Modified the frequency_limit entry for rw-access to allow using the common OMAP SGX sysfs interface for GPU overclock.

Supported by Trickster Mod Kernel Settings App.

As the pvr-source has been moved from the kernel source we have to add some additional changes in the OMAP OPP source code.
omap: gpu: add store function to frequency_limit
omap: gpu: enable 416 mhz sgx speed
omap: gpu: revert alternate sgx speed to original value