imximage: Add support for i.MX6

The i.MX6 processor can boot from NOR flash and SATA disks,
additionally. Add the flash offsets for these additional
boot modes.

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