ravb: Document PFRI register bit

Document PFRI register bit, as it is documented on R-Car Gen3 and
RZ/G2L hardware manuals.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Suggested-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/renesas/ravb.h

@@ -828,7 +828,7 @@ enum ECSR_BIT {
 	ECSR_MPD	= 0x00000002,
 	ECSR_LCHNG	= 0x00000004,
 	ECSR_PHYI	= 0x00000008,
-	ECSR_PFRI	= 0x00000010,
+	ECSR_PFRI	= 0x00000010,	/* Documented for R-Car Gen3 and RZ/G2L */
 };
 
 /* ECSIPR */