README.imximage: Fix the maximum DCD size

In commit 021e79c8 ("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>