hw/ppc/pnv_xscom: Rename pnv_xscom_realize(Error **) -> pnv_xscom_init()

pnv_xscom_realize() is not used to *realize* QDev object, rename
it as pnv_xscom_init(). The Error** argument is unused: remove it.

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: <20231019131647.19690-3-philmd@linaro.org>