mpc85xx: Add cmdline override patch

This patch adds a new kernel option called CONFIG_CMDLINE_OVERRIDE. This
setting is for devices with locked down u-boot environments, where users
are unable to change the default bootargs. When set, the fdt driver will
propagate the cmdline for the kernel from chosen/bootargs-override
instead of chosen/bootargs as long as it exists within the DTB.

Signed-off-by: Chris Blake <chrisrblake93@gmail.com>