Document -march=gfx906 option.

2019-06-07  Andrew Stubbs  <ams@codesourcery.com>

	gcc/
	* doc/invoke.texi (AMD GCN Options): Add gfx906.

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 3103f86ce98..fa7d9ea8100 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -16211,6 +16211,9 @@ Compile for GCN3 Fiji devices (gfx803).
 @item gfx900
 Compile for GCN5 Vega 10 devices (gfx900).
 
+@item gfx906
+Compile for GCN5 Vega 20 devices (gfx906).
+
 @end table
 
 @item -mstack-size=@var{bytes}