media: platform: renesas: rzg2l-cru: Add missing documentation for image_conv_irq

Add missing documentation for image_conv_irq element in
struct rzg2l_cru_dev.

Documentation needed to avoid build warning with W=1 builds. It doesn't
really add any non obvious information but good to have it anyway.

Reported-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>