docs: clarify Rockchip silicon — PineTab2 is RK3566, not RK3568

Surfaced during iter7 Track F research: the campaign target hardware
is Rockchip RK3566 silicon (PineTab2). The hantro driver attaches
via the rockchip,rk3568-vpu DT compatible because the RK3566 silicon
is close enough to RK3568 to share that variant. The proper RK3566
mainline driver target (rkvdec2 / vdpu346) has no kernel support yet
— Christian Hewitt's patch series LKML 2025/12/26/206 is unmerged.

Updates the two src/ comments that called the hardware "RK3568":
- context.c: hantro-vpu device-init S_EXT_CTRLS comment now reads
  "via rockchip,rk3568-vpu DT compatible (covers RK3568 and RK3566
  — PineTab2 silicon — since they're close enough)"
- h264.c: DPB pic_num discussion ends "...never surfaced on PineTab2
  (RK3566 via hantro/rk3568-vpu)"

Not a correctness change. Compiles + decodes identically. The
update matters for upstream submission accuracy (bootlin/Rockchip
maintainers will care which silicon the campaign tested on).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/context.c

@@ -131,7 +131,9 @@ VAStatus RequestCreateContext(VADriverContextP context, VAConfigID config_id,
 	 * device fd (request_fd=-1) before VIDIOC_STREAMON; per-request
 	 * controls (SPS/PPS/etc.) attached to a request_fd come later.
 	 *
-	 * hantro-vpu (RK3568) accepts only DECODE_MODE_FRAME_BASED.
+	 * hantro-vpu via rockchip,rk3568-vpu DT compatible (covers RK3568
+	 * and RK3566 — PineTab2 silicon — since they're close enough)
+	 * accepts only DECODE_MODE_FRAME_BASED.
 	 * START_CODE_ANNEX_B preserves leading 0x00000001 in the slice
 	 * payload that h264.c assembles. Errors here are not fatal: not
 	 * every backing driver supports both controls (e.g. cedrus may

src/h264.c

@@ -294,7 +294,7 @@ static void h264_fill_dpb(struct request_data *data,
 		 * one consumer that fails on that. Hantro doesn't read
 		 * pic_num at all (uses reference_ts for ref resolution),
 		 * which is why fourier's wrong value never surfaced on
-		 * RK3568.
+		 * PineTab2 (RK3566 via hantro/rk3568-vpu).
 		 */
 		if (entry->pic.flags & VA_PICTURE_H264_LONG_TERM_REFERENCE) {
 			dpb->pic_num = entry->pic.frame_idx;