soc: silabs: Drop useless comments

Most of the Kconfig files in soc/silabs start with a comment describing
the chip. This comment is redundant with the help message in associated
to the chip. Let's don't repeat ourselves.

This patch also ensure the Kconfig help message the full name the chip.

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