media: mtk-vpu: add description for wdt fields at struct mtk_vpu

Fix those warnings:
  drivers/media/platform/mtk-vpu/mtk_vpu.c:223: warning: No description found for parameter 'wdt'
  drivers/media/platform/mtk-vpu/mtk_vpu.c:223: warning: No description found for parameter 'wdt_refcnt'

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>