ARM: dts: am335x-chili*: Move uart0 description from SOM to board

uart0 configuration code has been in SOM. However, it is possible to
use all (or none) of 6 uart's of AM335x processor present on ChiliSOM.

This fix moves declaration of uart0 from ChiliSOM to ChiliBoard, because
use of uart is strictly board-specific.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>