dt-bindings: clock: sifive: add FU540-C000 PRCI clock constants

Add preprocessor macros for the important PRCI output clocks
that are needed by both the FU540 PRCI driver and DT data.
Details are available in the FU540 manual in Chapter 7 of

    https://static.dev.sifive.com/FU540-C000-v1.0.pdfSigned-off-by: NPaul Walmsley <paul.walmsley@sifive.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>

include/dt-bindings/clock/sifive-fu540-prci.h

+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2018-2019 SiFive, Inc.
+ * Wesley Terpstra
+ * Paul Walmsley
+ */
+
+#ifndef __DT_BINDINGS_CLOCK_SIFIVE_FU540_PRCI_H
+#define __DT_BINDINGS_CLOCK_SIFIVE_FU540_PRCI_H
+
+/* Clock indexes for use by Device Tree data and the PRCI driver */
+
+#define PRCI_CLK_COREPLL	       0
+#define PRCI_CLK_DDRPLL		       1
+#define PRCI_CLK_GEMGXLPLL	       2
+#define PRCI_CLK_TLCLK		       3
+
+#endif