STMOD_CELLULAR: add DISCO_L562_QE support

Author: jeromecoutant

components/cellular/COMPONENT_STMOD_CELLULAR/README.md

@@ -121,6 +121,20 @@ Default HW configuration:
 
 BG96 expansion board can be used but without flow control.
 
+### DISCO_L562_QE
+
+https://os.mbed.com/platforms/ST-Discovery-L562QE/
+
+HW configuration:
+
+Depending on json configuration, STMOD pins 1, 2, 3 and 4 can be mapped to SPI3 or UART3.
+
+        "config": { "stmod_sel": { "help": "1 for UART / 0 for SPI - see board User Manual, STMOD_1/2/3/4 are configurable",
+
+Default choice in targets.json is UART
+
+BG96 expansion board is then fully compatible.
+
 ### DISCO_H747I
 
 https://os.mbed.com/platforms/ST-Discovery-H747I/