docs: Add skeletal documentation of highbank and midway

Add skeletal documentation for the highbank and midway machines.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210713142226.19155-4-peter.maydell@linaro.org
diff --git a/MAINTAINERS b/MAINTAINERS
index 1504951..4256ad1 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -643,6 +643,7 @@
 S: Odd Fixes
 F: hw/arm/highbank.c
 F: hw/net/xgmac.c
+F: docs/system/arm/highbank.rst
 
 Canon DIGIC
 M: Antony Pavlov <antonynpavlov@gmail.com>