imx8: add dummy clock

This driver is mostly used to avoid build errors.
We use uclass clk driver for clk related operations.
Signed-off-by: NPeng Fan <peng.fan@nxp.com>
Reviewed-by: NAnatolij Gustschin <agust@denx.de>
Cc: Stefano Babic <sbabic@denx.de>

arch/arm/include/asm/arch-imx8/clock.h

+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright 2018 NXP
+ */
+
+#ifndef __ASM_ARCH_IMX8_CLOCK_H__
+#define __ASM_ARCH_IMX8_CLOCK_H__
+
+/* Mainly for compatible to imx common code. */
+enum mxc_clock {
+	MXC_ARM_CLK = 0,
+	MXC_AHB_CLK,
+	MXC_IPG_CLK,
+	MXC_UART_CLK,
+	MXC_CSPI_CLK,
+	MXC_AXI_CLK,
+	MXC_DDR_CLK,
+	MXC_ESDHC_CLK,
+	MXC_ESDHC2_CLK,
+	MXC_ESDHC3_CLK,
+	MXC_I2C_CLK,
+	MXC_FEC_CLK,
+};
+
+u32 mxc_get_clock(enum mxc_clock clk);
+
+#endif /* __ASM_ARCH_IMX8_CLOCK_H__ */

arch/arm/mach-imx/imx8/clock.c

+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright 2018 NXP
+ */
+
+#include <common.h>
+#include <linux/errno.h>
+#include <asm/arch/clock.h>
+
+DECLARE_GLOBAL_DATA_PTR;
+
+u32 mxc_get_clock(enum mxc_clock clk)
+{
+	switch (clk) {
+	default:
+		printf("Unsupported mxc_clock %d\n", clk);
+		break;
+	}
+
+	return 0;
+}