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>
diff --git a/doc/board/emulation/index.rst b/doc/board/emulation/index.rst
index a09ead1..be66b6b 100644
--- a/doc/board/emulation/index.rst
+++ b/doc/board/emulation/index.rst
@@ -8,6 +8,7 @@
 
    qemu-arm
    qemu-mips
+   qemu-ppce500
    qemu-riscv
    qemu-x86
    qemu_capsule_update