Support uncompressing zImage which is larger than 8M.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Peter Robinson <pbrobinson@gmail.com>
Cc: Volodymyr Riazantsev <volodymyr.riazantsev@globallogic.com>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Adrian Alonso <aalonso@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>