powerpc: xics_teardown_cpu()

[John Rose]

This function currently makes an eoi call that unconditionally sets the
CPPR bits to 0xff.  For the case of CPU DLPAR remove, the firmware
require those bits to be 0x00.  We explicitly set the CPPR to 0xff
earlier in the function.

Any opinions on whether we should:
1) Reorder xics_teardown_cpu() to set the CPPR after the eoi call
2) Add another call to zero out the CPPR at the end of teardown_cpu()
3) Add a new hcall (H_IPOLL) wrapper to determine the current CPPR, and
use that in the eoi call.

Any thoughts would be greatly appreciated!

Thanks-
John