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>
diff --git a/maintainers.rst b/maintainers.rst
index 94bb5d3..c01d97a 100644
--- a/maintainers.rst
+++ b/maintainers.rst
@@ -20,6 +20,8 @@
 -----------------------------
 :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/