ARM: dts: Add missing wdt3 node for omap4

On omap4 we're missing the wdt3 node with it's related "ti,hwmods"
property that the SoC interconnect code needs.

Note that this will only show up as a bug with "doesn't have
mpu register target base" boot errors when the legacy platform
data is removed.

Cc: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>