top: Rename "plat" to "platform" everywhere

This patch renames "plat" to "platform" everywhere for better
readablility.

Signed-off-by: Anup Patel <anup.patel@wdc.com>

platform/common/include/plat/serial/sifive-uart.h

@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2018 Western Digital Corporation or its affiliates.
+ *
+ * Authors:
+ *   Anup Patel <anup.patel@wdc.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#ifndef __SERIAL_SIFIVE_UART_H__
+#define __SERIAL_SIFIVE_UART_H__
+
+#include <sbi/sbi_types.h>
+
+void sifive_uart_putc(char ch);
+
+char sifive_uart_getc(void);
+
+int sifive_uart_init(unsigned long base,
+		     u32 in_freq, u32 baudrate);
+
+#endif