dts: arm: silabs: Move xg1 socs to xg1 directory

Introduce subdirectory for xg1 socs.
As a porduct of this - `efm32_pg_1b.dtsi` and `efr32fg1p.dtsi` were
merged into `xg1.dtsi`.

Signed-off-by: Yishai Jaffe <yishai1999@gmail.com>