ipq806x: improve system latency

ipq806x: improve system latency

Various report and data show that the freq 384000 is too low and cause some
extra latency to the entire system. OEM qsdk code also set the min frequency
for this target to 800 mhz.
Also some user notice some instability with this idle frequency, solved by
setting the min frequency to 600mhz. Fix all these kind of problem by
introducing a boot init.d script that set the min frequency to 600mhz and set
the ondemand governor to be more aggressive. The script set these value only if
the ondemand governor is detected. 384 mhz freq is still available and user can
decide to restore the old behavior by disabling this script.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>