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>

doc/README.imximage

@@ -120,7 +120,7 @@ Configuration command line syntax:
 				DATA 4 0x73FA88a0 0x200
 
 The processor support up to 60 register programming commands for IMXIMAGE_VERSION 1
-and 121 register programming commands for IMXIMAGE_VERSION 2.
+and 220 register programming commands for IMXIMAGE_VERSION 2.
 An error is generated if more commands are found in the configuration file.
 
 3. All commands are optional to program.