powerpc/mpc8548cds: Remove incorrect DDR_MSYNC_IN erratum define

This erratum doesn't exist on this processor, and the workaround
spins on a non-existent register, causing boot to hang.

Signed-off-by: Becky Bruce <beckyb@kernel.crashing.org>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>