hw/ppc/pnv_psi: Use device_cold_reset() instead of device_legacy_reset()

hw/ppc/pnv_psi: Use device_cold_reset() instead of device_legacy_reset()

The pnv_psi.c code uses device_legacy_reset() for two purposes:
 * to reset itself from its qemu_register_reset() handler
 * to reset a XiveSource object it has

Neither it nor the XiveSource have any qbuses, so the new
device_cold_reset() function (which resets both the device and its
child buses) is equivalent here to device_legacy_reset() and we can
just switch to the new API.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20210503151849.8766-4-peter.maydell@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>