xtensa: add MX irqchip

MX is an interrupt distributor used in some SMP-capable xtensa
configurations.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
xtensa: add SMP support

This is largely based on SMP code from the xtensa-2.6.29-smp tree by
Piet Delaney, Marc Gauthier, Joe Taylor, Christian Zankel (and possibly
other Tensilica folks).

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
xtensa: add Three Core HiFi-2 MX Variant.

This variant is equipped with MX and is capable of running SMP linux.

Signed-off-by: Piet Delaney <piet.delaney@gmail.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
xtensa: implement CPU hotplug

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>