ARM: dts: add support for AM437x StarterKit

Add support for TI's AM437x StarterKit Evaluation
Module.

Cc: Josh Elliot <jelliott@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Tested-by: Franklin Cooper Jr. <fcooper@ti.com>
Tested-by: Tom Rini <trini@ti.com>
Tested-by: Darren Etheridge <detheridge@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>