STM32F0 - Enable the low power timer

Enable the low power timer for the following targets:
 - NUCLEO_F070RB
 - NUCLEO_F072RB
 - NUCLEO_F091RC

Author: svastm

hal/targets.json

@@ -615,7 +615,7 @@
         "inherits": ["Target"],
         "progen": {"target": "nucleo-f030r8"},
         "detect_code": ["0725"],
-        "device_has": ["ANALOGIN", "I2C", "I2CSLAVE", "INTERRUPTIN", "LOWPOWERTIMER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "RTC", "SERIAL", "SLEEP", "SPI", "SPISLAVE", "STDIO_MESSAGES"],
+        "device_has": ["ANALOGIN", "I2C", "I2CSLAVE", "INTERRUPTIN", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "RTC", "SERIAL", "SLEEP", "SPI", "SPISLAVE", "STDIO_MESSAGES"],
         "default_lib": "small",
         "release_versions": ["2"]
     },