media: mtk-vpu/mtk_vpu.h: fix kernel-doc warnings

A top-level documentation block was missing 'DOC:'.

Fix typos in vpu_wdt_reg_handler() and vpu_mapping_dm_addr().

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: Irui Wang <irui.wang@mediatek.com>
Cc: Andrew-CT Chen <andrew-ct.chen@mediatek.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>