board/qemu/ppc64le-pseries: put Qemu command on one line

This allows the toolchain building machinery used by
https://toolchains.bootlin.com

 to automatically re-use this Qemu
command line.

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