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/docs/plat/allwinner.rst b/docs/plat/allwinner.rst
index 825fded..140edf5 100644
--- a/docs/plat/allwinner.rst
+++ b/docs/plat/allwinner.rst
@@ -22,12 +22,18 @@
 or the environment variable BL31 must contain the binary's path.
 See the respective `U-Boot documentation`_ for more details.
 
-To build:
+To build for machines with an A64 or H5 SoC:
 
 ::
 
     make CROSS_COMPILE=aarch64-linux-gnu- PLAT=sun50i_a64 DEBUG=1 bl31
 
+To build for machines with an H6 SoC:
+
+::
+
+    make CROSS_COMPILE=aarch64-linux-gnu- PLAT=sun50i_h6 DEBUG=1 bl31
+
 .. _U-Boot documentation: http://git.denx.de/?p=u-boot.git;f=board/sunxi/README.sunxi64;hb=HEAD
 
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