[POWERPC] Separate IRQ config / register set from main header

There is no need to expose these settings outside the scope
of the interrupt controller code itself.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Paul Mackerras <paulus@samba.org>