These days, the NACA only exists on iSeries.  Therefore, this patch
moves naca.h from include/asm-ppc64 to arch/powerpc/platforms/iseries.
There was one file including naca.h outside of platforms/iseries -
arch/ppc64/kernel/udbg_scc.c.  However, that's obviously a hangover
from older days.  The include is not necessary, so this patch simply
removes it.

Built and booted on iSeries, built for G5 (which uses udbg_scc.o).
Signed-off-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>