boards: silabs: Add chipset reference to "sltb004a"

Usually commercial names mention the chipset included on the board.
"Thunderboard Sense 2" is the only exception in the Silabs boards.

This information is probably useful, so add it to the description of
this board.

Signed-off-by: Jérôme Pouiller <[email protected]>

Author: jerome-pouiller

boards/silabs/dev_kits/sltb004a/sltb004a.yaml

@@ -1,5 +1,5 @@
 identifier: sltb004a
-name: Thunderboard Sense 2 (SLTB004A, BRD4166A)
+name: Thunderboard Sense 2 (SLTB004A, BRD4166A, EFR32MG12)
 type: mcu
 arch: arm
 ram: 256