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>

board/qemu/ppc64le-pseries/readme.txt

@@ -1,6 +1,6 @@
 Run the emulation with:
 
-qemu-system-ppc64 -M pseries -nographic -kernel output/images/vmlinux -initrd output/images/rootfs.ext2
+qemu-system-ppc64 -M pseries -cpu POWER8 -m 256 -kernel output/images/vmlinux -append "console=hvc0 root=/dev/sda" -drive file=output/images/rootfs.ext2,if=scsi,index=0,format=raw -serial stdio -display curses
 
 The login prompt will appear in the terminal window.