m68k: Remove M54455EVB board

This board has not been converted to CONFIG_DM_MMC by the deadline.
Remove it.

Cc: Angelo Durgehello <angelo.dureghello@timesys.com>
Cc: TsiChung Liew <Tsi-Chung.Liew@nxp.com>
Signed-off-by: Tom Rini <trini@konsulko.com>