Add some spiXdr doc

SpinalHDL/SpinalHDL#1263

source/SpinalHDL/Libraries/Com/spiXdr.rst

@@ -0,0 +1,56 @@
+
+SPI XDR
+========
+
+There is a SPI controller which support :
+
+- half/full duplex
+- single/dual/quad SPI
+- SDR/DDR/.. data rate
+
+
+You can find its APB3 implementation here : 
+
+https://github.com/SpinalHDL/SpinalHDL/blob/68b6158700fc2440ea7980406f927262c004faca/lib/src/main/scala/spinal/lib/com/spi/xdr/Apb3SpiXdrMasterCtrl.scala#L43
+
+Configuration
+----------------------------
+
+Here is an example.
+
+.. code-block:: scala
+
+      Apb3SpiXdrMasterCtrl(
+        SpiXdrMasterCtrl.MemoryMappingParameters(
+          SpiXdrMasterCtrl.Parameters(
+            dataWidth = 8, // Each transfer will be 8 bits
+            timerWidth = 12, // The timer is used to slow down the transmition
+            spi = SpiXdrParameter( //Specify the physical SPI interface
+              dataWidth = 4, //Number of physical SPI data pins
+              ioRate = 1, //Specify the number of transfer that each spi pin can do per clock 1 => SDR, 2 => DDR
+              ssWidth = 1 //Number of chip selects
+            )
+          )
+          .addFullDuplex(id = 0) // Add support for regular SPI (MISO / MOSI) using the mode id 0
+          .addHalfDuplex( // Add another mode
+            id = 1,  // mapped on mode id 1
+            rate = 1, // When rate is 1, the clock will do up to one toggle per cycle, divided by the (timer+1)
+                      // When rate bigger (ex 2), the controller will ignore the timer, and use the SpiXdrParameter.ioRate
+                      // capabilities to emit up to "rate" transition per clock cycle.
+            ddr = false, // sdr => 1 bit per SPI clock, DDR => 2 bits per SPI clock
+            spiWidth = 4 //Number of physical SPI data pin used for serialisation
+          ),
+          cmdFifoDepth = 32,
+          rspFifoDepth = 32,
+          xip = null
+        )
+      )
+
+Software Driver
+----------------------------
+
+See :
+
+https://github.com/SpinalHDL/SaxonSoc/blob/dev-0.3/software/standalone/driver/spi.h
+https://github.com/SpinalHDL/SaxonSoc/blob/dev-0.3/software/standalone/spiDemo/src/main.c
+