User controllable CPU speed option

Useful as a workaround for games that lag after a while due to changing CPU frequencies often(see #2104), or to experiment with. It doesn't seem to break any games that I have, so here it is. It's off by default to allow PPSSPP to have its typical behaviour.

To use it: edit ppsspp.ini's CPUSpeed = 0 line to whatever you want the CPU speed to be locked to. If the option is unchecked and re-enabled at a later date, it defaults to 222mhz.