Enable reading core/h264/v3d/isp frequency minimum from system config if set

[trabant-asb]

In version 0.5.5, the minimum clock frequencies for the GPU cells are hard-coded to 250. This PRs approach is to first see if core_freq_min/h264_freq_min/v3d_freq_min/isp_freq_min is set, if not, check if gpu_freq_min is set, if none of them, default do 250.

Also, there is a change in the limits used for colorization for V3D as I found it is always active (like the Core) and not just on demand (as i.e. the h264).

Fixes #24

Let me know if further information is needed.