drm/exynos/exynos_drm_gsc: Supply missing description for 'num_limits'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/exynos/exynos_drm_gsc.c:129: warning: Function parameter or member 'num_limits' not described in 'gsc_driverdata'

Cc: Inki Dae <inki.dae@samsung.com>
Cc: Joonyoung Shim <jy0922.shim@samsung.com>
Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: Eunchul Kim <chulspro.kim@samsung.com>
Cc: Jinyoung Jeon <jy0.jeon@samsung.com>
Cc: Sangmin Lee <lsmin.lee@samsung.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-samsung-soc@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201116174112.1833368-7-lee.jones@linaro.org

drivers/gpu/drm/exynos/exynos_drm_gsc.c

@@ -118,6 +118,7 @@ struct gsc_context {
  * struct gsc_driverdata - per device type driver data for init time.
  *
  * @limits: picture size limits array
+ * @num_limits: number of items in the aforementioned array
  * @clk_names: names of clocks needed by this variant
  * @num_clocks: the number of clocks needed by this variant
  */