docs: Add documentation of Arm 'kzm' board

Add brief documentation of the Arm 'kzm' board.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210722175229.29065-3-peter.maydell@linaro.org
3 files changed