ARM: mach-imx: Remove board entries in dt_board_compat

There is no need for adding board related entries into dt_board_compat.

Leave only the SoC entry.

This way we do not need to patch a C file when adding dt support for
a new board.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>