Add FPGA_FORCE_ALL_PORTS example

Author: gpsimenos

hal/tests/TESTS/mbed_hal_fpga_ci_test_shield/README.md

@@ -10,6 +10,10 @@ mbed test -n "hal-tests-tests*fpga*" --app-config TESTS/configs/fpga.json
 
 The `FPGA_FORCE_ALL_PORTS` macro can be defined to force all peripherals to be tested. Some FPGA tests only test one by default, to save time.
 
+```
+mbed test -n "hal-tests-tests*fpga*" --app-config TESTS/configs/fpga.json -DFPGA_FORCE_ALL_PORTS
+```
+
 FPGA_CI_TEST_SHIELD needed macro  
 and specific test capabilities per target  
 are defined in: