media: v4l2-mem2mem: add descriptions to MC fields

drivers/media/v4l2-core/v4l2-mem2mem.c:90: warning: Function parameter or member 'source_pad' not described in 'v4l2_m2m_dev'
drivers/media/v4l2-core/v4l2-mem2mem.c:90: warning: Function parameter or member 'sink' not described in 'v4l2_m2m_dev'
drivers/media/v4l2-core/v4l2-mem2mem.c:90: warning: Function parameter or member 'sink_pad' not described in 'v4l2_m2m_dev'
drivers/media/v4l2-core/v4l2-mem2mem.c:90: warning: Function parameter or member 'proc' not described in 'v4l2_m2m_dev'
drivers/media/v4l2-core/v4l2-mem2mem.c:90: warning: Function parameter or member 'proc_pads' not described in 'v4l2_m2m_dev'
drivers/media/v4l2-core/v4l2-mem2mem.c:90: warning: Function parameter or member 'intf_devnode' not described in 'v4l2_m2m_dev'

Fixes: be2fff65 ("media: add helpers for memory-to-memory media controller")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>