ADD: add documentation on SimUDP

sim_udp.md

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+
+# Simulated User Datagram Protocol (SimUDP)
+
+The SimUDP peripheral provides a mean to transmit and receive datagrams through a UDP socket connection. 
+
+## SimUDP Registers
+
+### USART RXIP Register
+
+Address offset: 0x00
+Reset value: 0x0100_007F
+
+`Bit 31:0` IP: The IP address to listen to, in network endianness (big-endian)
+
+### USART TXIP Register
+
+Address offset: 0x04
+Reset value: 0x0100_007F
+
+`Bit 31:0` IP: The IP address to send to, in network endianness (big-endian)
+
+### USART RXPORT Register
+
+Address offset: 0x08
+Reset value: 0x0000_0000
+
+`Bit 15:0` PORT: The receive port in network endianness (big-endian)
+
+### USART TXPORT Register
+
+Address offset: 0x0C
+Reset value: 0x0000_0000
+
+`Bit 15:0` PORT: The transmit port in network endianness (big-endian)
+
+### USART CTRL Register
+
+Address offset: 0x10
+Reset value: 0x0000_0000
+
+`Bit 0` INIT: Initialize the socket object. Writing 1 to this bit creates a new socket.
+
+`Bit 1` RXEN: Enable the reception functionality. 
+
+`Bit 2` TXEN: Enable the transmission functionality. 
+
+`Bit 3` ST: Start tranmission of a Datagram. Writing 1 to this bit starts the transmitsion of a Datagram. After transmission, this bit will be cleared by hardware.
+
+### USART STATUS Register
+
+Address offset: 0x14
+Reset value: 0x0000_0000
+
+`Bit 15:0` MAXRXSIZE: Maximum number of bytes to receive.
+
+### USART RXSIZE Register
+
+Address offset: 0x18
+Reset value: 0x0000_0000
+
+`Bit 15:0` SIZE: Number of bytes received.
+
+### USART TXSIZE Register
+
+Address offset: 0x1C
+Reset value: 0x0000_0000
+
+`Bit 15:0` SIZE: Number of bytes to transmit.
+
+### USART RXFIFO Register[0:255]
+
+Address offset: 0x30
+Reset value: 0x0000_0000
+
+`Bit 31:0` DATA
+
+### USART TXSIZE Register[0:255]
+
+Address offset: 0x130
+Reset value: 0x0000_0000
+
+`Bit 31:0` DATA
+