docs/clocks: add device's clock documentation

docs/clocks: add device's clock documentation

Add the documentation about the clock inputs and outputs in devices.

This is based on the original work of Frederic Konrad.

Signed-off-by: Damien Hedde <damien.hedde@greensocs.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 20200406135251.157596-6-damien.hedde@greensocs.com
[PMM: Editing pass for minor grammar, style and Sphinx
 formatting fixes]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>