imx6 / wandboard SMP

[Ian Lepore]

Thanks to an excellent patchset contributed by Juergen Weiss we now have
SMP for Wandboard and imx6 in general.  

To test this yourself, update to r262587 or later and add "option SMP"
to your kernel config.

I haven't yet committed kernel config changes to add "option SMP"
because I didn't want to take anyone by surprise on an update.  It seems
to be pretty stable in the limited testing I've done, but I stress
"limited" (I haven't even tested sdcard yet, just nfsroot).

I committed the bulk of the work last weekend, and today I committed the
last bits, involving using the WFI instruction in the idle loop and
doing some minimal setup of the imx6 power control to keep the cores
powered on enough to process timer interrupts during WFI.  More on that
in a separate message.

So a big round of thanks to Juergen for the SMP work, and to Steven
Lawrance for the temperature-sensor stuff so we can be sure the SMP
stuff isn't cooking our chips.

-- Ian