Add a function for checking whether a clock has a source.  This is
useful for devices which have input clocks that must be wired up by
the board as it allows them to fail in realize rather than ploughing
on with a zero-period clock.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210128114145.20536-3-peter.maydell@linaro.org
Message-id: 20210121190622.22000-3-peter.maydell@linaro.org