powerpc/mpc8536ds: Save the env variables to SDcard and SPI flash

MPC8536DS offer booting from SDcard or SPI flash.  This patch defined that
u-boot can save the environment variables on SDcard or SPI flash when
booting from the related device.  The Env parameter region and linux
kernel region have overlap in SPI-Flash, So change the Env param saving
address.

Signed-off-by: Xie Xiaobo <r63061@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>