plat: gxl: Add documentation on building GXL image

Also adds a maintainer for GXL.

Signed-off-by: Remi Pommarel <repk@triplefau.lt>
diff --git a/readme.rst b/readme.rst
index 41ffc0f..6f47812 100644
--- a/readme.rst
+++ b/readme.rst
@@ -211,6 +211,7 @@
 
 -  Allwinner sun50i_a64 and sun50i_h6
 -  Amlogic Meson S905 (GXBB)
+-  Amlogic Meson S905x (GXL)
 -  Arm Juno Software Development Platform
 -  Arm Neoverse N1 System Development Platform (N1SDP)
 -  Arm Neoverse Reference Design N1 Edge (RD-N1-Edge) FVP