armsse: Wire up systick cpuclk clock

Wire up the cpuclk for the systick devices to the SSE object's
existing mainclk clock.

We do not wire up the refclk because the SSE subsystems do not
provide a refclk.  (This is documented in the IoTKit and SSE-200
TRMs; the SSE-300 TRM doesn't mention it but we assume it follows the
same approach.) When we update the systick device later to honour "no
refclk connected" this will fix a minor emulation inaccuracy for the
SSE-based boards.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Message-id: 20210812093356.1946-8-peter.maydell@linaro.org