allwinner: Add Allwinner H6 SoC support

The H6 is Allwinner's most recent SoC. It shares most peripherals with the
other ARMv8 Allwinner SoCs (A64/H5), but has a completely different memory
map.

Introduce a separate platform target, which includes a different header
file to cater for the address differences. Also add the new build target
to the documentation.

The new ATF platform name is "sun50i_h6".

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/plat/allwinner/common/sunxi_bl31_setup.c b/plat/allwinner/common/sunxi_bl31_setup.c
index e910ee5..8ecf490 100644
--- a/plat/allwinner/common/sunxi_bl31_setup.c
+++ b/plat/allwinner/common/sunxi_bl31_setup.c
@@ -74,6 +74,9 @@
 	case 0x1718:
 		soc_name = "H5";
 		break;
+	case 0x1728:
+		soc_name = "H6";
+		break;
 	default:
 		soc_name = "unknown";
 		break;