Rework the cpu frequency management code for imx6.

This adds the concept of "operating points," combinations of frequency
and voltage at which the cpu is known to work correctly.  Some day these
should come from FDT data, but for now the table is hard-coded.

This also allows tuning the min and max operating frequencies.  The min
frequency is what the thermal management code will slow down to if the
core temperature gets too high.  The max frequency is what gets used if
the temperature is okay.

Normally the max cannot be set higher than the value burned into the
ocotp fuses as the chip's rated max, but there is now a new sysctl+tunable
cpu_overclock_enable; when set to non-zero it allows raising the frequency
above the ocotp value: USE WITH CARE!  (At least one of my imx6 boards
has a cpu whose ocotp values never got set correctly; they claim a max
of 792mhz, but the physical markings on the chip say it's good to 1ghz.)

Because all these values affect the entire SoC, there is a new sysctl
node, hw.imx6, where all these values live.  The values that are currently
under dev.imx6_anatop.0 should probably move to hw.imx6 too, because
"anatop" doesn't even mean anything to me, let alone to an end user.