board/qemu/ppc64le-pseries: use qemu-system-ppc64

qemu-system-ppc64le doesn't necessarily exist: it isn't installed by
Qemu, and only created as a symlink to qemu-system-ppc64 by some
distributions (Ubuntu). Other distributions (Fedora) just have
qemu-system-ppc64.

But qemu-system-ppc64 is capable of running little-endian PPC64
systems, so use this one instead.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>