arm: Remove mx53smd board

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

Cc: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>