ppc: Remove Cyrus_P5020 and P5040 boards

These boards have not been converted to CONFIG_DM_MMC by the deadline.
Remove them.  As the P5020 is the last ARCH_P5020 platform, remove that
support as well.

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