arm: fix bootm with device tree

Commit 0a672d49

 "arm: Add Prep subcommand support to bootm" re-organized
do_bootm_linux() for ARM. During the re-organization, the code to pass
the device tree to the kernel was removed. Add it back. This restores
the ability to boot a kernel using device tree.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Tom Rini <trini@ti.com>
Acked-by: Allen Martin <amartin@nvidia.com>
Tested-by: Allen Martin <amartin@nvidia.com>