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>
diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 04c5e9c..c049c066 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -137,7 +137,7 @@
 	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.