pl110: Clarify comment about PL110 ID on VersatilePB

Clarify a comment about the ID register value presented by
the PL110 variant present on the VersatilePB board (based
on testing what the actual hardware does), to indicate that
this is not an error in our emulation, and to remove an #if-0.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

hw/display/pl110.c

@@ -94,23 +94,21 @@ static const VMStateDescription vmstate_pl110 = {
 static const unsigned char pl110_id[] =
 { 0x10, 0x11, 0x04, 0x00, 0x0d, 0xf0, 0x05, 0xb1 };
 
-/* The Arm documentation (DDI0224C) says the CLDC on the Versatile board
-   has a different ID.  However Linux only looks for the normal ID.  */
-#if 0
-static const unsigned char pl110_versatile_id[] =
-{ 0x93, 0x10, 0x04, 0x00, 0x0d, 0xf0, 0x05, 0xb1 };
-#else
-#define pl110_versatile_id pl110_id
-#endif
-
 static const unsigned char pl111_id[] = {
     0x11, 0x11, 0x24, 0x00, 0x0d, 0xf0, 0x05, 0xb1
 };
 
+
 /* Indexed by pl110_version */
 static const unsigned char *idregs[] = {
     pl110_id,
-    pl110_versatile_id,
+    /* The ARM documentation (DDI0224C) says the CLCDC on the Versatile board
+     * has a different ID (0x93, 0x10, 0x04, 0x00, ...). However the hardware
+     * itself has the same ID values as a stock PL110, and guests (in
+     * particular Linux) rely on this. We emulate what the hardware does,
+     * rather than what the docs claim it ought to do.
+     */
+    pl110_id,
     pl111_id
 };