dts: provide stm32 soc dtsi files for stm32 base boards

This commit provides dtsi files for available stm32 base boards.
For now only uart nodes and IRQ number are provided in order to
enable delivery of coherent material.

It also clears additional content from stm32f103xb.dtsi

Change-Id: I62d932c7f22b56e95bcd9566ce39e14a393dd640
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>