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>