hw/ppc/pnv_xscom: Move sysbus_mmio_map() call within pnv_xscom_init()

In order to make the next commit trivial, move sysbus_init_mmio()
calls just before the corresponding sysbus_mmio_map() calls.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20231019131647.19690-4-philmd@linaro.org>