ARM: uniphier: support Debug UART

For ARM32 architecture, CONFIG_DEBUG_LL is available for early
low-level debugging (and actually UniPhier 32bit SoCs use it), but
ARM64 architecture does not support it.  Instead, CONFIG_DEBUG_UART
is available as an architecture-independent debug facility.

This commit supports it on all the UniPhier SoCs (including the new
ARMv8 SoCs), which is very useful for new SoC bringups.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index 92d4212..2a770a1 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -169,6 +169,15 @@
 	  will need to provide parameters to make this work. The driver will
 	  be available until the real driver model serial is running.
 
+config DEBUG_UART_UNIPHIER
+	bool "UniPhier on-chip UART"
+	depends on ARCH_UNIPHIER
+	help
+	  Select this to enable a debug UART using the UniPhier on-chip UART.
+	  You will need to provide DEBUG_UART_BASE to make this work.  The
+	  driver will be available until the real driver-model serial is
+	  running.
+
 endchoice
 
 config DEBUG_UART_BASE