ppc/pnv: change initrd address

When skiboot starts, it first clears the CPU structs for all possible
CPUs on a system :

	for (i = 0; i <= cpu_max_pir; i++)
		memset(&cpu_stacks[i].cpu, 0, sizeof(struct cpu_thread));

On POWER9, cpu_max_pir is quite big, 0x7fff, and the skiboot cpu_stacks
array overlaps with the memory region in which QEMU maps the initramfs
file. Move it upwards in memory to keep it safe.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>