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/uart8250.h

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2018 Western Digital Corporation or its affiliates.
+ *
+ * Authors:
+ *   Anup Patel <anup.patel@wdc.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#ifndef __SERIAL_UART8250_H__
+#define __SERIAL_UART8250_H__
+
+#include <sbi/sbi_types.h>
+
+void uart8250_putc(char ch);
+
+char uart8250_getc(void);
+
+int uart8250_init(unsigned long base,
+		  u32 in_freq, u32 baudrate,
+		  u32 reg_shift, u32 reg_width);
+
+#endif