initial description of Raspberry Pi 5 support

doc/hosts/raspberry-pi-5.md

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+# Running LiteX-M2SDR on the Raspberry Pi 5
+
+The M2SDR is connected to the Raspberry Pi 5 through the M2 HAT found at
+https://www.kubii.com/fr/raspberry-pi-5/4114-m2-hat-plus-5056561803463.html
+
+<img src="raspberry-pi-5.jpg">
+
+Using the PCI board providing JTAG access on the USB-C connector, flash the 
+``litex_m2sdr_m2_pcie_operational.zip`` bitstream found
+at https://github.com/enjoy-digital/litex_m2sdr/issues/30 using 
+```
+openFPGALoader -c ft4232 --fpga-part xc7a200tsbg484 --freq 20e6 -f litex_m2sdr_m2_pcie.bin 
+```
+
+## Using the binary Raspberry Pi OS
+
+Use ``kernel=kernel8.img`` in accordance with
+https://github.com/raspiblitz/raspiblitz/issues/4346 for using 4 KB memory page
+size rather than the default kernel using 16 KB memory page size which confuses
+DMA allocation.
+
+Append the ``config.txt`` in the first FAT partition with
+```
+[cm5]
+dtoverlay=dwc2,dr_mode=host
+
+kernel=kernel8.img
+
+[all]
+dtparam=pciex1=on
+dtoverlay=pcie-32bit-dma-pi5
+dtoverlay=pciex1-compat-pi5,l1ss=off,no-l0s=on,no-mip=off
+```
+
+Append at the end of the ``cmdline.txt`` in the fist FAT partition the Linux 
+kernel options
+```
+pci=noaer pcie_aspm=off iommu.passthrough=1 arm-smmu.disable=1
+```
+
+## Testing with M2SDR tools
+
+Follow the testing sequence described at 
+https://github.com/enjoy-digital/litex_m2sdr/issues/18 and ending, if all tests
+are positive, with
+
+```
+./m2sdr_rf --rx_freq 102.4e6 --rx_gain 70 --samplerate 5e6
+./m2sdr_record /tmp/out.bin 750000000
+```
+which is postprocessed with the GNU Radio flowchart
+
+<img src="FM_postprocess.png">
+
+identifying how data are stored as interleaved complex short (16 bit) integers.
+
+## Testing with GNU Radio
+
+The [following flowchart](raspberry-pi-5_FM.grc) is used to demonstrate 
+continuous data acquisition at 5 Msamples/s and real time decoding of the 
+commercial wideband analog FM signal using the M2SDR Soapy source. 
+Tune to your local station frequency.
+
+<img src="raspberry-pi-5_FM.png">