ARM:vt8500: Convert to use .restart and remove arch_reset()

Removed system.h as it only contained an inline for arch_reset()
Changed the existing board files to use .restart in there machine
descriptions.

Added device tree support for the restart controller.
Device tree support for mach-vt8500 is still a work-in-progress.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>