PPC [liftoff]: add cp to the offset documentation

Change-Id: Icd9d2c1c7c6b9e1ce2ebbf078e8984beda8c258d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3212898Reviewed-by: Junliang Yan <junyan@redhat.com>
Commit-Queue: Milad Fa <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/main@{#77306}

src/wasm/baseline/ppc/liftoff-assembler-ppc.h

@@ -27,9 +27,10 @@ namespace liftoff {
 //   3   |   parameter 0      | or parameter 1
 //   2   |  (result address)  | or parameter 0
 //  -----+--------------------+---------------------------
-//   1   | return addr (lr)   |
-//   0   | previous frame (fp)|
-//  -----+--------------------+  <-- frame ptr (fp)
+//   2   | return addr (lr)   |
+//   1   | previous frame (fp)|
+//   0   | const pool (r28)   | if const pool is enabled
+//  -----+--------------------+  <-- frame ptr (fp) or cp
 //  -1   | 0xa: WASM          |
 //  -2   |     instance       |
 //  -3   |    feedback vector |