arm: rmobile: Add Silicon Linux EK874 board support

The EK874 development kit from Silicon Linux is made of CAT874 (the main
board) and CAT875 (the sub board that goes on top of CAT874).

This patch adds the required board support to boot Si-Linux EK874 board
based on R8A774C0 SoC.

DTS files apart from r8a774c0-ek874-u-boot.dts and r8a774c0-u-boot.dtsi
have been imported from Linux kernel 5.11 commit f40ddce88593
("Linux 5.11").

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
diff --git a/include/configs/silinux-ek874.h b/include/configs/silinux-ek874.h
new file mode 100644
index 0000000..25c0cd2
--- /dev/null
+++ b/include/configs/silinux-ek874.h
@@ -0,0 +1,20 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * include/configs/silinux-ek874.h
+ *     This file is Silicon Linux EK874 board configuration.
+ *
+ * Copyright (C) 2021 Renesas Electronics Corporation
+ */
+
+#ifndef __SILINUX_EK874_H
+#define __SILINUX_EK874_H
+
+#include "rcar-gen3-common.h"
+
+/* Ethernet RAVB */
+#define CONFIG_BITBANGMII_MULTI
+
+/* Generic Timer Definitions (use in assembler source) */
+#define COUNTER_FREQUENCY	0xFE502A	/* 16.66MHz from CPclk */
+
+#endif /* __SILINUX_EK874_H */