docs/system: Briefly document canon-a1100 board

Add skeletal documentation of the canon-a1100 board.

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: 20200713175746.5936-2-peter.maydell@linaro.org