configs/qemu: update arm-vexpress instructions for SMP

We can use the vexpress arm defconfig to test SMP features in Qemu.
So document the necessary invocation changes to account for this, in
preparation of the arm nuri defconfig removal.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

board/qemu/arm-vexpress/readme.txt

@@ -1,8 +1,11 @@
 Run the emulation with:
 
-  qemu-system-arm -M vexpress-a9 -m 256 -kernel output/images/zImage -dtb output/images/vexpress-v2p-ca9.dtb -drive file=output/images/rootfs.ext2,if=sd,format=raw -append "console=ttyAMA0,115200 root=/dev/mmcblk0" -serial stdio -net nic,model=lan9118 -net user
+  qemu-system-arm -M vexpress-a9 -smp 1 -m 256 -kernel output/images/zImage -dtb output/images/vexpress-v2p-ca9.dtb -drive file=output/images/rootfs.ext2,if=sd,format=raw -append "console=ttyAMA0,115200 root=/dev/mmcblk0" -serial stdio -net nic,model=lan9118 -net user
 
 The login prompt will appear in the terminal that started Qemu. The
 graphical window is the framebuffer.
 
+If you want to emulate more cores change "-smp 1" to "-smp 2" for
+dual-core or even "smp -4" for a quad-core configuration.
+
 Tested with QEMU 2.3.0