hw/timer/armv7m_systick: Add input clocks

The v7M systick timer can be programmed to run from either of
two clocks:
 * an "external reference clock" (when SYST_CSR.CLKSOURCE == 0)
 * the main CPU clock (when SYST_CSR.CLKSOURCE == 1)

Our implementation currently hardwires the external reference clock
to be 1MHz, and allows boards to set the main CPU clock frequency via
the global 'system_clock_scale'.  (Most boards set that to a constant
value; the Stellaris boards allow the guest to reprogram it via the
board-specific RCC registers).

As the first step in converting this to use the Clock infrastructure,
add input clocks to the systick device for the reference clock and
the CPU clock.  The device implementation ignores them; once we have
made all the users of the device correctly wire up the new Clocks we
will switch the implementation to use them and ignore the old
system_clock_scale.

This is a migration compat break for all M-profile boards, because of
the addition of the new clock objects to the vmstate struct.

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: 20210812093356.1946-6-peter.maydell@linaro.org

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