arm: rmobile: silk: Migrate serial driver to drivers model

This adds drivers model support of serial port to Silk board,
and migrate serial port to drivers model.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
diff --git a/configs/silk_defconfig b/configs/silk_defconfig
index 515ee33..3130e93 100644
--- a/configs/silk_defconfig
+++ b/configs/silk_defconfig
@@ -1,3 +1,5 @@
 CONFIG_ARM=y
 CONFIG_RMOBILE=y
 CONFIG_TARGET_SILK=y
+CONFIG_DM=y
+CONFIG_DM_SERIAL=y