feat(pm): select XTAL for HP dig_sysclk in modem state

On ESP32-S31, SOC and modem APB are decoupled, so the digital
system can run from XTAL while the modem domain stays in modem state.

components/soc/esp32s31/include/soc/Kconfig.soc_caps.in

@@ -1539,6 +1539,10 @@ config SOC_PM_SUPPORT_PMU_MODEM_STATE
     bool
     default y
 
+config SOC_PM_MODEM_STATE_USE_XTAL
+    bool
+    default y
+
 config SOC_PM_PMU_MIN_SLP_SLOW_CLK_CYCLE_FIXED
     bool
     default y

components/soc/esp32s31/include/soc/soc_caps.h

@@ -572,6 +572,8 @@
 #define SOC_PM_MODEM_RETENTION_BY_REGDMA           (1)
 #define SOC_PM_SUPPORT_PMU_MODEM_STATE             (1)
 #define MAC_SUPPORT_PMU_MODEM_STATE                SOC_PM_SUPPORT_PMU_MODEM_STATE
+/* Since SOC APB and MODEM APB on ESP32-S31 are decoupled, XTAL can be used as SOC clock in modem state */
+#define SOC_PM_MODEM_STATE_USE_XTAL                (1)
 
 /*
  * min_slp_val is derived from slow clock period (450us / slow_clk).