hw/arm/stellaris: Wire sysclk up to armv7m

Connect the sysclk to the armv7m object.  This board's SoC does not
connect up the systick reference clock, so we don't need to connect a
refclk.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alexandre Iooss <erdnaxe@crans.org>
Message-id: 20210812093356.1946-18-peter.maydell@linaro.org