allwinner: Add Samuel as second maintainer

As Samuel contributed most of the original code and he has an interest
in that, add him as a second maintainer for the Allwinner port.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>

maintainers.rst

@@ -20,6 +20,8 @@ Allwinner ARMv8 platform port
 -----------------------------
 :M: Andre Przywara <andre.przywara@arm.com>
 :G: `Andre-ARM`_
+:M: Samuel Holland <samuel@sholland.org>
+:G: `smaeul`_
 :F: docs/plat/allwinner.rst
 :F: plat/allwinner/