ARM: sun9i: smp: Support CPU/cluster power down and hotplugging for cpu1~7

This patch adds common code used to power down all cores and clusters.
The code was previously based on the MCPM framework. It has now been
adapted to hook into struct smp_operations directly, but the code
structure still shows signs of prior work.

The primary core (cpu0) requires setting flags to have the BROM bounce
execution to the SMP software entry code. This is done in a subsequent
patch to keep the changes cleanly separated. By default the ARM SMP code
blocks cpu0 from being turned off, so splitting this out is safe.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>