Add Semtech Lora radio drivers

components/lora/COMPONENT_SX126X/SleepMode.txt

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+Sleep Modes:
+
+The SX126X series LoRa radios define two different sleep modes, namely:
+
+i) Sleep mode with Cold Start (default mode in Mbed LoRaWAN stack)
+ii) Sleep mode with Warm Start
+
+
+Sleep mode with Warm Start:
+This is the default sleep mode for this driver. Radio configurations are retained in this mode. 
+Typical power consumption in this mode is '600 nA'.
+
+
+Sleep mode with Cold Start:
+The driver can be configured to sleep with cold startup. This mode is the lowest power consuming state 
+for the SX126X series radios. No configurations are retained in this mode, that's why our driver takes 
+extra measures to keep backups of the configuration in the RAM. Typical power consumption in this mode
+is '160 nA'. The radio takes about 3.5 milliseconds to wakeup properly because upon going to sleep all
+components gets turned off. The radio thread blocks for that period of time. However, to reduce the impact 
+of this wakeup time on the time critical operations, the stack shouldn't put the radio to standby rather than 
+sleep before performing time critical operations. Mbed OS LoRaWAN stack handles this automatically which means
+that the user can safely use sleep mode with cold start.    

components/lora/COMPONENT_SX126X/mbed_lib.json

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+{
+    "name": "SX126X-lora-driver",
+    "config": {
+        "spi-frequency": {
+        	"help": "SPI frequency, Default: 16 MHz",
+        	"value": 16000000
+        },
+        "buffer-size": {
+        	"help": "Max. buffer size the radio can handle, Default: 255 B",
+        	"value": 255
+        },
+        "boost-rx": {
+        	"help": "Increases sensitivity at the cost of power ~2mA for around ~3dB in sensitivity 0 = disabled, 1 = enabled",
+        	"value": 0
+        },
+        "regulator-mode": {
+        	"help": "Default: DCDC (low power, high BOM). Alternatively, LDO = 0. Check datasheet section 5.1 for more details",
+        	"value": 1
+        },
+        "sleep-mode": {
+        	"help": "Default: Cold start = 1, Warm start = 0. Check SleepMode.txt",
+        	"value": 1
+        },
+        "standby-mode": {
+        	"help": "Default: STDBY_RC = 0, STDBY_XOSC = 1",
+        	"value": 0
+        }
+    }
+}