ppc/pnv: Support for SECURITY_SWITCH XSCOM register access

Power Hypervisor code requires access to the SECURITY_SWITCH
XSCOM register at MMIO address 0x80028 (scom address 0x10005).
Adding basic read support for now so that is doesn't cause
error messages to be posted.

Reviewed-by: Chalapathi V <chalapathi.v@linux.ibm.com>
Reviewed-by: Glenn Miles <milesg@linux.ibm.com>
Reviewed-by: Aditya Gupta <adityag@linux.ibm.com>
Signed-off-by: Glenn Miles <milesg@linux.ibm.com>
Signed-off-by: Caleb Schlossin <calebs@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20260210134647.2050821-2-calebs@linux.ibm.com
Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>

hw/ppc/pnv_xscom.c

@@ -61,6 +61,8 @@ static uint32_t pnv_xscom_pcba(PnvChip *chip, uint64_t addr)
 static uint64_t xscom_read_default(PnvChip *chip, uint32_t pcba)
 {
     switch (pcba) {
+    case 0x10005:       /* SECURITY SWITCH */
+        return 0;
     case 0xf000f:
         return PNV_CHIP_GET_CLASS(chip)->chip_cfam_id;
     case 0x18002:       /* ECID2 */