media: admin-guide: add stm32-dma2d description

add stm32-dma2d description for dma2d driver

Signed-off-by: Dillon Min <dillon.minfei@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>