DTS: bindings: omap: update bindings documentation

* clarify that we now need either "ti,omap3430" or "ti,omap3630" or "ti,am3517" for omap3 chips
* clarify that "ti,omap3" has no default
* clarify that AM33x is not an "ti,omap3"
* clarify that the list of boards is incomplete
* remove some "ti,am33xx", "ti,omap3"
* add some missing "ti,omap4"

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Tested-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>