MIPS: zboot: Add support for serial debug using the PROM

As most platforms implement the PROM serial interface prom_putchar()
add a simple bridge to allow re-using this code for zboot.

Signed-off-by: Alban Bedel <albeu@free.fr>
Cc: Alex Smith <alex.smith@imgtec.com>
Cc: Andrew Bresticker <abrestic@chromium.org>
Cc: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/11811/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Kconfig

@@ -1766,6 +1766,10 @@ config SYS_SUPPORTS_ZBOOT_UART16550
 	bool
 	select SYS_SUPPORTS_ZBOOT
 
+config SYS_SUPPORTS_ZBOOT_UART_PROM
+	bool
+	select SYS_SUPPORTS_ZBOOT
+
 config CPU_LOONGSON2
 	bool
 	select CPU_SUPPORTS_32BIT_KERNEL

arch/mips/boot/compressed/Makefile

@@ -35,6 +35,7 @@ vmlinuzobjs-y := $(obj)/head.o $(obj)/decompress.o $(obj)/string.o
 ifdef CONFIG_DEBUG_ZBOOT
 vmlinuzobjs-$(CONFIG_DEBUG_ZBOOT)		   += $(obj)/dbg.o
 vmlinuzobjs-$(CONFIG_SYS_SUPPORTS_ZBOOT_UART16550) += $(obj)/uart-16550.o
+vmlinuzobjs-$(CONFIG_SYS_SUPPORTS_ZBOOT_UART_PROM) += $(obj)/uart-prom.o
 vmlinuzobjs-$(CONFIG_MIPS_ALCHEMY)		   += $(obj)/uart-alchemy.o
 endif
 

arch/mips/boot/compressed/uart-prom.c

@@ -0,0 +1,7 @@
+
+extern void prom_putchar(unsigned char ch);
+
+void putc(char c)
+{
+	prom_putchar(c);
+}