ARM: at91/soc: add basic support for new sama5d2 SoC

Add Kconfig entries, header file changes and addition to the documentation.
The early debug infrastructure is also added for easy development.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Olof Johansson <olof@lixom.net>
diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
index a2e16f9..946c8c0 100644
--- a/arch/arm/Kconfig.debug
+++ b/arch/arm/Kconfig.debug
@@ -141,6 +141,12 @@
 		depends on ARCH_AT91
 		depends on SOC_SAMA5
 
+	config AT91_DEBUG_LL_DBGU3
+		bool "Kernel low-level debugging on sama5d2"
+		select DEBUG_AT91_UART
+		depends on ARCH_AT91
+		depends on SOC_SAMA5
+
 	config DEBUG_BCM2835
 		bool "Kernel low-level debugging on BCM2835 PL011 UART"
 		depends on ARCH_BCM2835