ARM: dts: da850-lego-ev3: Add node for reboot modes

This adds a new node for reboot modes on LEGO MINDSTORMS EV3. The EV3's
bootloader looks for a magic number in the ARM local RAM and if found,
it will boot into a special firmware update mode where the flash memory
can be written via USB.

This has been testing working using the command:

    # systemctl reboot loader

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>