doc: board: raspberrypi: Add documentation

Add documentation about the different configuration files for the
RaspberryPi board family.

Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/board/index.rst b/doc/board/index.rst
index be9ba4d..f7bfc44 100644
--- a/doc/board/index.rst
+++ b/doc/board/index.rst
@@ -14,6 +14,7 @@
    apple/index
    armltd/index
    atmel/index
+   broadcom/index
    congatec/index
    coreboot/index
    emulation/index