imx: Fix build of mx6sxsabresd

Commit 224beb833e544b802f08765271cec07667d39669 add clock
enabling function for FEC, but the masks are not available
for SX processor and the mx6sxsabresd cannot be built clean.

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Fabio Estevam <fabio.estevam@freescale.com>
CC: Nikita Kiryanov <nikita@compulab.co.il>
README.imximage: Fix the maximum DCD size

In commit 021e79c85371 ("tools: imximage: Fix the maximum DCD size for
mx53/mx6") we have fixed the maximum DCD size for mx53/mx53.

Do the same on the README document for consistency.

Reported-by: Jonas Karlsson <jonas.d.karlsson@gmail.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
imx: mx6q/dlarm2: Change to use generic board

Enable the CONFIG_SYS_GENERIC_BOARD for imx6q/dl arm2 board to
use generic board.

Signed-off-by: Ye.Li <B37916@freescale.com>
imx: mx6slevk: Change to use generic board

Enable CONFIG_SYS_GENERIC_BOARD for imx6slevk to use generic board.

Signed-off-by: Ye.Li <B37916@freescale.com>