ppc: Remove MPC8569MDS board

This board has not been converted to CONFIG_DM_MMC by the deadline.
Remove it.  As this is the last ARCH_MPC8569 board, remove that support
as well.

Cc: Priyanka Jain <priyanka.jain@nxp.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>