ppc: qemu: Move board directory from board/freescale to board/emulation

board/emulation is the place for other QEMU targets like x86, arm,
riscv. Let's move the qemu-ppce500 board codes there.

List me as a co-maintainer for this board.

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