serial: uniphier: move CONFIG_UNIPHIER_SERIAL to Kconfig

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index 6a392ba..a0b6e02 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -4,3 +4,9 @@
 	help
 	  If you want to use driver model for serial drivers, say Y.
 	  To use legacy serial drivers, say N.
+
+config UNIPHIER_SERIAL
+	bool "UniPhier on-chip UART support"
+	depends on ARCH_UNIPHIER && DM_SERIAL
+	help
+	  Support for the on-chip UARTs on the Panasonic UniPhier platform.