esp_modem: Add option which enables UART_ISR_IN_IRAM to support OTA

Closes https://github.com/espressif/esp-idf/issues/4582

examples/protocols/pppos_client/main/Kconfig.projbuild

@@ -54,6 +54,15 @@ menu "Example Configuration"
                 Enter the peer phone number that you want to send message to.
     endif
 
+    config EXAMPLE_UART_ISR_IN_RAM
+        bool "Use UART ISR in RAM"
+        default n
+        select UART_ISR_IN_IRAM
+        help
+            Enable placing UART ISR to IRAM.
+            This options is useful if PPPoS client needs to perform
+            flash operations (e.g. OTA) while keeping the network operational.
+
     config EXAMPLE_LCP_ECHO
         bool "Enable LCP ECHO"
         default n