imximage: Add MX53 to the documentation

i.MX53 is supported, too. Add this to the introduction of the
documentation.

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
CC: Fabio Estevam <festevam@gmail.com>
CC: Stefano Babic <sbabic@denx.de>
CC: Jason Liu <jason.hui@linaro.org>

doc/README.imximage

@@ -3,7 +3,7 @@ Imximage Boot Image generation using mkimage
 ---------------------------------------------
 
 This document describes how to set up a U-Boot image
-that can be booted by Freescale MX25, MX35 and MX51
+that can be booted by Freescale MX25, MX35, MX51 and MX53
 processors via internal boot mode.
 
 These processors can boot directly from NAND, SPI flash and SD card flash