tools: imximage: Fix the maximum DCD size for mx53/mx6

tools: imximage: Fix the maximum DCD size for mx53/mx6

According to mx53 and mx6 reference manuals:

"The maximum size of the DCD limited to 1768 bytes."

As each DCD entry consists of 8 bytes, we have a total of 1768 / 8 = 221, and
excluding the first entry, which is the header leads to 220 as the maximum
number for DCD size.

Reported-by: Jonas Karlsson <jonas.d.karlsson@gmail.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Nitin Garg <nitin.garg@freescale.com>
Acked-by: Nitin Garg <nitin.garg@freescale.com>