add SPI driver for semtech sx128x chips

sx128x/README.md

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+# SX128x Radio
+Radio from Semtech in the 2.4 GHz band. This driver uses SPI to communicate with the radio instead of the alternative UART interface.
+
+## Supported Chips
+- [SX1280](https://www.semtech.com/products/wireless-rf/lora-connect/sx1280)
+- [SX1281](https://www.semtech.com/products/wireless-rf/lora-connect/sx1281) 
+

sx128x/commands.go

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+package sx128x
+
+const (
+	// SX128X SPI commands
+	CMD_NOP = uint8(0x00)
+
+	CMD_SET_UART_SPEED = uint8(0x9D)
+	CMD_GET_STATUS     = uint8(0xC0)
+
+	// Register Access Operations
+	CMD_WRITE_REGISTER = uint8(0x18)
+	CMD_READ_REGISTER  = uint8(0x19)
+
+	// Data Buffer Operations
+	CMD_WRITE_BUFFER = uint8(0x1A)
+	CMD_READ_BUFFER  = uint8(0x1B)
+
+	// Radio Operation Modes
+	CMD_SET_SLEEP                  = uint8(0x84)
+	CMD_SET_STANDBY                = uint8(0x80)
+	CMD_SET_FS                     = uint8(0xC1)
+	CMD_SET_TX                     = uint8(0x83)
+	CMD_SET_RX                     = uint8(0x82)
+	CMD_SET_RX_DUTY_CYCLE          = uint8(0x94)
+	CMD_SET_LONG_PREAMBLE          = uint8(0x9B)
+	CMD_SET_CAD                    = uint8(0xC5)
+	CMD_SET_TX_CONTINUOUS_WAVE     = uint8(0xD1)
+	CMD_SET_TX_CONTINUOUS_PREAMBLE = uint8(0xD2)
+	CMD_SET_AUTO_TX                = uint8(0x98)
+	CMD_SET_AUTO_FS                = uint8(0x9E)
+
+	// Radio Configuration
+	CMD_SET_PACKET_TYPE         = uint8(0x8A)
+	CMD_GET_PACKET_TYPE         = uint8(0x03)
+	CMD_SET_RF_FREQUENCY        = uint8(0x86)
+	CMD_SET_TX_PARAMS           = uint8(0x8E)
+	CMD_SET_CAD_PARAMS          = uint8(0x88)
+	CMD_SET_BUFFER_BASE_ADDRESS = uint8(0x8F)
+	CMD_SET_MODULATION_PARAMS   = uint8(0x8B)
+	CMD_SET_PACKET_PARAMS       = uint8(0x8C)
+
+	// Communication Status Information
+	CMD_GET_RX_BUFFER_STATUS = uint8(0x17)
+	CMD_GET_PACKET_STATUS    = uint8(0x1D)
+	CMD_GET_RSSI_INST        = uint8(0x1F)
+
+	// IRQ Handling
+	CMD_SET_DIO_IRQ_PARAMS = uint8(0x8D)
+	CMD_GET_IRQ_STATUS     = uint8(0x15)
+	CMD_CLEAR_IRQ_STATUS   = uint8(0x97)
+
+	// Miscellaneous
+	CMD_SET_REGULATOR_MODE = uint8(0x96)
+	CMD_SET_SAVE_CONTEXT   = uint8(0xD5)
+)