power9 shared caches support

[Oliver O'Halloran]

This is a pretty substantial rework of the last patch that should address most of the comment mpe had, namely:

1. Removed the random cpu_feature(ARCH_300) check. The new series always builds the cache mask and the new scheduler topology is used if we detect a mismatch of the thread and 

2. In the previous series the online and offlining paths were symmetrical. Althoug this is nice in theory it resulted in a bit unnecessary indirection. To simplify things it now only parse the device-tree, etc in the onlining path. When offlining a CPU all we need to do is walk the masks of the offlining CPU.

There's also fixes for various nitpicks.