LiveOPP: add PLLDDR boost interface

Thanks @cocafe for idea and original patch.
I found way to overclock PLLDDR stably upto 975 MHz (on my codina).
To use this interface type command like that:
`echo 0x0005017f > /sys/kernel/liveopp/pllddr # 975 MHz`
Please note only values that starts with 0x0005.... are allowed.