CODEOWNERS: add codeowner of stamp_c3

Add myself as codeowner of previously committed boards.

- #53328

Signed-off-by: TOKITA Hiroshi <[email protected]>

Author: soburi

CODEOWNERS

@@ -183,6 +183,7 @@
 /boards/riscv/neorv32/                    @henrikbrixandersen
 /boards/riscv/niosv*/                     @sweeaun
 /boards/riscv/sparkfun_red_v_things_plus/ @soburi
+/boards/riscv/stamp_c3/                   @soburi
 /boards/shields/                          @erwango
 /boards/shields/atmel_rf2xx/              @nandojve
 /boards/shields/esp_8266/                 @nandojve