Convert CONFIG_KEYBOARD to Kconfig

This converts the following to Kconfig:
   CONFIG_KEYBOARD

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/Kconfig b/arch/Kconfig
index 3e2cc84..fffddac 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -194,6 +194,7 @@
 	imply PHY_FIXED
 	imply DM_DSA
 	imply CMD_EXTENSION
+	imply KEYBOARD
 
 config SH
 	bool "SuperH architecture"