drivers: modem_cellular: Add autostart support for BG95

Add optional autostart boolean to the quectel,bg95 binding and make
MODEM_CELLULAR_DEFINE_INSTANCE() use it through DT_PROP_OR().
Boards that carry a BG95-M3 Mini-PCIe card—or any other variant that
boots at VCC can now declare the property and skip the PWRKEY pulse,
while existing designs continue to behave unchanged.

Signed-off-by: Van Petrosyan <van.petrosyan@sensirion.com>

Author: petrosyan-van

drivers/modem/modem_cellular.c

@@ -2635,7 +2635,7 @@ MODEM_CHAT_SCRIPT_DEFINE(sqn_gm02s_periodic_chat_script,
 		.reset_pulse_duration_ms = (reset_ms),                                             \
 		.startup_time_ms  = (startup_ms),                                                  \
 		.shutdown_time_ms = (shutdown_ms),                                                 \
-		.autostarts       = (start),                                                       \
+		.autostarts = DT_PROP_OR(DT_DRV_INST(inst), autostart, (start)),                   \
 		.set_baudrate_chat_script    = (set_baudrate_script),                              \
 		.init_chat_script            = (init_script),                                      \
 		.dial_chat_script            = (dial_script),                                      \

dts/bindings/modem/quectel,bg95.yaml

@@ -10,3 +10,11 @@ include: uart-device.yaml
 properties:
   mdm-power-gpios:
     type: phandle-array
+
+  autostart:
+    type: boolean
+    description: |
+      Set for modem variants or carrier boards that start the module
+      automatically when supply voltage is applied—that is, the modem
+      asserts PWRKEY internally and does not require an external PWRKEY
+      pulse.