hw/sysbus: Inline and remove sysbus_add_io()

hw/sysbus: Inline and remove sysbus_add_io()

sysbus_add_io(...) is a simple wrapper to
memory_region_add_subregion(get_system_io(), ...).
It is used in 3 places; inline it directly.

Rationale: we want to move to an explicit I/O bus,
rather that an implicit one. Besides in heterogeneous
setup we can have more than one I/O bus.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20240216150441.45681-1-philmd@linaro.org>
[PMD: Include missing "exec/address-spaces.h" header]
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>