soc: silabs: Fix SoC names

As discussed here[1], CONFIG_SOC should rely on CONFIG_SOC_* variables
while it currently rely on CONFIG_SOC_PART_NUMBER_*. These variable are in
fact misnamed since the migration to HWMv2. So "PART_NUMBER" has to be
removed from these names.

[1]: https://github.com/zephyrproject-rtos/

 \
     zephyr/pull/93285#discussion_r2221382981

Note, this patch has been generated with:

    sed -i -e s/SOC_PART_NUMBER_MGM/SOC_MGM/   \
           -e s/SOC_PART_NUMBER_EF/SOC_EF/     \
           -e s/SOC_PART_NUMBER_SIM3/SOC_SIM3/ \
           -e s/SOC_PART_NUMBER_SIWG/SOC_SIWG/ \
           soc/**/*.soc boards/**/Kconfig.*

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>