ppc: Remove MPC8315ERDB board

This board has not been converted to CONFIG_DM_MMC by the deadline.
Remove it.  As this is the last ARCH_MPC8315 platform, remove that
support as well.

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