dts/arm/silabs: update partition definitions

- To link image loadable by MCUboot, zephyr,code-partition
must be set in the DTS.
- Move partition definitions from SoC DTS to the board DTS.
- Remove scratch partition since MCUboot does not recommend to use it.
- Increase bootloader partitions to 48K to fit recent MCUboot.

Signed-off-by: Roman Dobrodii <rdobrodii@antmicro.com>