MIPS: ralink: adds early_printk support

Add the code needed to make early printk work.
Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4897/

arch/mips/ralink/early_printk.c

+/*
+ *  This program is free software; you can redistribute it and/or modify it
+ *  under the terms of the GNU General Public License version 2 as published
+ *  by the Free Software Foundation.
+ *
+ *  Copyright (C) 2011-2012 Gabor Juhos <juhosg@openwrt.org>
+ */
+
+#include <linux/io.h>
+#include <linux/serial_reg.h>
+
+#include <asm/addrspace.h>
+
+#define EARLY_UART_BASE         0x10000c00
+
+#define UART_REG_RX             0x00
+#define UART_REG_TX             0x04
+#define UART_REG_IER            0x08
+#define UART_REG_IIR            0x0c
+#define UART_REG_FCR            0x10
+#define UART_REG_LCR            0x14
+#define UART_REG_MCR            0x18
+#define UART_REG_LSR            0x1c
+
+static __iomem void *uart_membase = (__iomem void *) KSEG1ADDR(EARLY_UART_BASE);
+
+static inline void uart_w32(u32 val, unsigned reg)
+{
+	__raw_writel(val, uart_membase + reg);
+}
+
+static inline u32 uart_r32(unsigned reg)
+{
+	return __raw_readl(uart_membase + reg);
+}
+
+void prom_putchar(unsigned char ch)
+{
+	while ((uart_r32(UART_REG_LSR) & UART_LSR_THRE) == 0)
+		;
+	uart_w32(ch, UART_REG_TX);
+	while ((uart_r32(UART_REG_LSR) & UART_LSR_THRE) == 0)
+		;
+}