allwinner: Detect and output current SoC

So far we already support booting on two different SoCs, and we will
shortly add a third, so add some code to determine the current SoC type.
This can be later used to runtime detect certain properties.

Also print the SoC name to the console, to give valuable debug information.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/plat/allwinner/common/sunxi_private.h b/plat/allwinner/common/sunxi_private.h
index b9f0fb4..e45f494 100644
--- a/plat/allwinner/common/sunxi_private.h
+++ b/plat/allwinner/common/sunxi_private.h
@@ -12,6 +12,7 @@
 void sunxi_cpu_on(unsigned int cluster, unsigned int core);
 void sunxi_disable_secondary_cpus(unsigned int primary_cpu);
 
+uint16_t sunxi_read_soc_id(void);
 void sunxi_security_setup(void);
 
 #endif /* __SUNXI_PRIVATE_H__ */