mako: support Kernel low-level debug via UART

- mako use UART4DM as debug UART.
- add the DEBUG_LL_UART in Kconfig.debug to choose UART DEBUG
  defined in other Kconfig

Change-Id: I1cbcd233ac739890fd08da25bba44a0717016607
diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
index ed18cae..1c010a8 100644
--- a/arch/arm/Kconfig.debug
+++ b/arch/arm/Kconfig.debug
@@ -265,6 +265,11 @@
 		  definition internally in order for low-level debugging to
 		  work.
 
+	config DEBUG_LL_UART
+		bool "low-level debugging UART"
+		help
+		  Say Y here if you want the debug print routines to UART
+
 	config DEBUG_ICEDCC
 		bool "Kernel low-level debugging via EmbeddedICE DCC channel"
 		help