dt-bindings: phy: phy-imx8-pcie: Add binding for the pad modes of imx8 pcie phy

Add binding for reference clock PAD modes of the i.MX8 PCIe PHY.
Signed-off-by: NRichard Zhu <hongxing.zhu@nxp.com>
Tested-by: NMarcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: NTim Harvey <tharvey@gateworks.com>
Tested-by: NTim Harvey <tharvey@gateworks.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1638432158-4119-2-git-send-email-hongxing.zhu@nxp.comSigned-off-by: NVinod Koul <vkoul@kernel.org>

include/dt-bindings/phy/phy-imx8-pcie.h

+/* SPDX-License-Identifier: (GPL-2.0+ OR MIT) */
+/*
+ * This header provides constants for i.MX8 PCIe.
+ */
+
+#ifndef _DT_BINDINGS_IMX8_PCIE_H
+#define _DT_BINDINGS_IMX8_PCIE_H
+
+/* Reference clock PAD mode */
+#define IMX8_PCIE_REFCLK_PAD_UNUSED	0
+#define IMX8_PCIE_REFCLK_PAD_INPUT	1
+#define IMX8_PCIE_REFCLK_PAD_OUTPUT	2
+
+#endif /* _DT_BINDINGS_IMX8_PCIE_H */