qdev: add clock input&output support to devices.

qdev: add clock input&output support to devices.

Add functions to easily handle clocks with devices.
Clock inputs and outputs should be used to handle clock propagation
between devices.
The API is very similar the GPIO API.

This is based on the original work of Frederic Konrad.

Signed-off-by: Damien Hedde <damien.hedde@greensocs.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20200406135251.157596-4-damien.hedde@greensocs.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>