doc: Add a reST document for qemu-ppce500

Add a reST document to describe how to build and run U-Boot for
the QEMU ppce500 machine.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>