dt-bindings: phy: Add constants for lan966x serdes

Lan966x has: 2 integrated PHYs, 3 SerDes and 2 RGMII interfaces. Which
requires to be muxed based on the HW representation.

So add constants for each interface to be able to distinguish them.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Link: https://lore.kernel.org/r/20211116100818.1615762-3-horatiu.vultur@microchip.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

include/dt-bindings/phy/phy-lan966x-serdes.h

+/* SPDX-License-Identifier: (GPL-2.0 OR MIT) */
+
+#ifndef __PHY_LAN966X_SERDES_H__
+#define __PHY_LAN966X_SERDES_H__
+
+#define CU(x)		(x)
+#define CU_MAX		CU(2)
+#define SERDES6G(x)	(CU_MAX + 1 + (x))
+#define SERDES6G_MAX	SERDES6G(3)
+#define RGMII(x)	(SERDES6G_MAX + 1 + (x))
+#define RGMII_MAX	RGMII(2)
+#define SERDES_MAX	(RGMII_MAX + 1)
+
+#endif