hw/arm/stellaris: split stellaris_sys_init()

Currently the stellaris_sys_init() function creates the
TYPE_STELLARIS_SYS object, sets its properties, realizes it, maps its
MMIO region and connects its IRQ.  In order to support wiring the
sysclk up to the armv7m object, we need to split this function apart,
because to connect the clock output of the STELLARIS_SYS object to
the armv7m object we need to create the STELLARIS_SYS object before
the armv7m object, but we can't wire up the IRQ until after we've
created the armv7m object.

Remove the stellaris_sys_init() function, and instead put the
create/configure/realize parts before we create the armv7m object and
the mmio/irq connection parts afterwards.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alexandre Iooss <erdnaxe@crans.org>
Message-id: 20210812093356.1946-17-peter.maydell@linaro.org