hw/core/clock: Add the clock_new helper function

hw/core/clock: Add the clock_new helper function

This function creates a clock and parents it to another object with a
given name. It calls clock_setup_canonical_path before returning the
new clock.

This function is useful to create clocks in devices when one doesn't
want to expose it at the qdev level (as an input or an output).

Suggested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Luc Michel <luc@lmichel.fr>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20201010135759.437903-4-luc@lmichel.fr>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>