docs/system: Document Musca boards

Provide a minimal documentation of the Musca boards.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200507151819.28444-6-peter.maydell@linaro.org
diff --git a/MAINTAINERS b/MAINTAINERS
index 520a7b7..023f48d 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -708,6 +708,7 @@
 L: qemu-arm@nongnu.org
 S: Maintained
 F: hw/arm/musca.c
+F: docs/system/arm/musca.rst
 
 Musicpal
 M: Jan Kiszka <jan.kiszka@web.de>