powerpc: fix annotation of fake_numa_create_new_node()

This function has always been marked as __cpuinit, but is only called
from functions marked as __init and references an __initdata variable.
So change its annotation to __init.

Fixes this build warning:

WARNING: arch/powerpc/mm/built-in.o(.cpuinit.text+0x86): Section mismatch in reference from the function .fake_numa_create_new_node() to the variable .init.data:cmdline
The function __cpuinit .fake_numa_create_new_node() references
a variable __initdata cmdline.
If cmdline is only used by .fake_numa_create_new_node then
annotate cmdline with a matching annotation.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>