ARM: OMAP2+: AM33xx: Add SoC specific restart hook

Add restart hook so that DTS based AM33xx builds can restart
the platform.

Tested-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Jean-Sebastien A. Beaudry <jsabeaudry@handyem.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>