ARM: davinci_all_defconfig: enable SYSCON_REBOOT_MODE

This enables SYSCON_REBOOT_MODE as a module. This is used by LEGO
MINDSTORMS EV3 to reboot into a special firmware update mode.

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