dts: arm: silabs: Remove use of CONFIG_SOC_* from Silabs SoC 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.  This lets us
remove:

CONFIG_SOC_PART_NUMBER_EFM32WG990F256
CONFIG_SOC_PART_NUMBER_EFR32FG1P133F256GM48

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