dts: arm: st: Remove use of CONFIG_SOC_* from STM32 L4 dts files

To move forward and remove use of Kconfig in dts files lets just create
SoC specific dtsi files that the boards can include.  We also seperate
out the L4 dtsi files into their own dir.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>