docs/system: Briefly document gumstix boards

Add skeletal documentation of the gumstix boards
('connex' and 'verdex').

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200713175746.5936-4-peter.maydell@linaro.org
diff --git a/MAINTAINERS b/MAINTAINERS
index 6973b68..935ccb3 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -652,6 +652,7 @@
 L: qemu-arm@nongnu.org
 S: Odd Fixes
 F: hw/arm/gumstix.c
+F: docs/system/arm/gumstix.rst
 
 i.MX25 PDK
 M: Peter Maydell <peter.maydell@linaro.org>