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