board/powerpc/ppc64le-pseries: set x-vof on qemu command line

This uses a newer firmware implementation that is much faster at
booting. It is supported as of Qemu 7.0.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

board/qemu/ppc64le-pseries/readme.txt

@@ -1,5 +1,5 @@
 Run the emulation with:
 
-qemu-system-ppc64 -M pseries -cpu POWER8 -m 256 -kernel output/images/vmlinux -append "console=hvc0 rootwait root=/dev/sda" -drive file=output/images/rootfs.ext2,if=scsi,index=0,format=raw -serial stdio -display curses # qemu_ppc64le_pseries_defconfig
+qemu-system-ppc64 -M pseries,x-vof=on -cpu POWER8 -m 256 -kernel output/images/vmlinux -append "console=hvc0 rootwait root=/dev/sda" -drive file=output/images/rootfs.ext2,if=scsi,index=0,format=raw -serial stdio -display curses # qemu_ppc64le_pseries_defconfig
 
 The login prompt will appear in the terminal window.