mx53smd: Convert to generic board

Enable CONFIG_SYS_GENERIC_BOARD, so that we get rid of the following warning on
boot:

"Warning: Your board does not use generic board. Please read
doc/README.generic-board and take action. Boards not
upgraded by the late 2014 may break or be removed."

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>