qemu_ppc64_e5500_defconfig: convert quotes in readme.txt emulation example

Resolves an error in the way the bootlin toolchain-builder uses
the board/qemu/ppc64-e5500/readme.txt to generate the qemu test
command.

https://github.com/free-electrons/toolchains-builder/blob/master/build.sh#L186

Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/qemu/ppc64-e5500/readme.txt

@@ -1,6 +1,6 @@
 Run the emulation with:
 
- qemu-system-ppc64 -M ppce500 -cpu e5500 -m 256 -kernel output/images/uImage -drive file=output/images/rootfs.ext2,if=virtio,format=raw -append 'console=ttyS0 root=/dev/vda' -serial mon:stdio -nographic
+ qemu-system-ppc64 -M ppce500 -cpu e5500 -m 256 -kernel output/images/uImage -drive file=output/images/rootfs.ext2,if=virtio,format=raw -append "console=ttyS0 root=/dev/vda" -serial mon:stdio -nographic
 
 The login prompt will appear in the terminal that started Qemu.