hw/arm/armv7m: Create input clocks

hw/arm/armv7m: Create input clocks

Create input clocks on the armv7m container object which pass through
to the systick timers, so that users of the armv7m object can specify
the clocks being used.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Message-id: 20210812093356.1946-7-peter.maydell@linaro.org