board/qemu/ppc64le-pseries: make Qemu command similar to ppc64-pseries

In particular:

 - Explicitly specify the CPU to be used, POWER8, which matches
   qemu_ppc64le_pseries_defconfig

 - Use hard disk emulation to access the root filesystem instead of an
   initrd.

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