tegra20: move SDRAM param save to later in boot

Move warmboot_save_sdram_params() to later in the boot sequence.  This
code relies on devicetree to get the address of the memory controller
and with upcoming changes for SPL boot it gets called early in the
boot process when devicetree is not initialized yet.

Signed-off-by: Allen Martin <amartin@nvidia.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Tested-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Tom Warren <twarren@nvidia.com>