doc: Set correct syntax highlighting style

Several code blocks do not specify a language for syntax
highlighting. This results in Sphinx using a default highlighter
which is Python.

This patch adds the correct language to each code block that doesn't
already specify it.

Change-Id: Icce1949aabfdc11a334a42d49edf55fa673cddc3
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
diff --git a/docs/plat/meson-gxl.rst b/docs/plat/meson-gxl.rst
index 3c39c9d..c6d8504 100644
--- a/docs/plat/meson-gxl.rst
+++ b/docs/plat/meson-gxl.rst
@@ -15,7 +15,7 @@
 
 In order to build it:
 
-::
+.. code:: shell
 
     CROSS_COMPILE=aarch64-linux-gnu- make DEBUG=1 PLAT=gxl