MIPS: ath79: Add zboot debug serial support

Reuse the early printk code to support the serial in zboot. We copy
early_printk.c instead of referencing it because we need to build a
different object file for the normal kernel and zboot.
Signed-off-by: Alban Bedel <albeu@free.fr>
Cc: Andrew Bresticker <abrestic@chromium.org>
Cc: Alex Smith <alex.smith@imgtec.com>
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/12234/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Kconfig

@@ -137,7 +137,7 @@ config ATH79
 	select SYS_SUPPORTS_32BIT_KERNEL
 	select SYS_SUPPORTS_BIG_ENDIAN
 	select SYS_SUPPORTS_MIPS16
-	select SYS_SUPPORTS_ZBOOT
+	select SYS_SUPPORTS_ZBOOT_UART_PROM
 	select USE_OF
 	help
 	  Support for the Atheros AR71XX/AR724X/AR913X SoCs.

arch/mips/boot/compressed/Makefile

@@ -37,8 +37,13 @@ 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)		   += $(obj)/uart-alchemy.o
+vmlinuzobjs-$(CONFIG_ATH79)			   += $(obj)/uart-ath79.o
 endif
 
+extra-y += uart-ath79.c
+$(obj)/uart-ath79.c: $(srctree)/arch/mips/ath79/early_printk.c
+	$(call cmd,shipped)
+
 vmlinuzobjs-$(CONFIG_KERNEL_XZ) += $(obj)/ashldi3.o $(obj)/bswapsi.o
 
 extra-y += ashldi3.c bswapsi.c