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.
