boards: silabs: Add internal board references

Yaml files allows the user to find the internal name for their board
using the "name" field.

In addition of the commercial names/references, Silabs boards have
internal board references. These references are not advertised, but can
be found printed on (every) boards.

It makes sense to add this reference to the Yaml file in case a user
would have the idea to grep it.

This patch also fixes some commercial names of very old boards.

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