Convert binding doc fsl-board.txt to yaml format. split to 3 part
fsl,bcsr.yaml, fsl,fpga-qixis.yaml, fsl,fpga-qixis-i2c.yaml

Additional change for fsl,fpga-qixis.yaml
- Add childnode mdio-mux-emi*
- Add compatible string fsl,ls1043aqds-fpga, fsl,ls1043ardb-fpga,
fsl,ls1046aqds-fpga, fsl,ls1046ardb-fpga, fsl,ls208xaqds-fpga,
fsl,ls1043ardb-cpld, fsl,ls1046ardb-cpld, fsl,ls1088aqds-fpga,
fsl,ls1088ardb-fpga, fsl,ls2080aqds-fpga, fsl,ls2080ardb-fpga.
- Change address to 32bit in example.

Additional change for fsl,fpga-qixis-i2c.yaml
- Add mux-controller
- Add compatible string fsl,ls1028aqds-fpga, fsl,lx2160aqds-fpga

Fix below warning:

arch/arm64/boot/dts/freescale/fsl-ls1028a-qds.dtb: /soc/i2c@2000000/fpga@66: failed to match any schema with compatible: ['fsl,ls1028aqds-fpga', 'fsl,fpga-qixis-i2c', 'simple-mfd']
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Link: https://lore.kernel.org/r/20240813163638.3889778-1-Frank.Li@nxp.com
[robh: tweak mdio-mux node name, drop .dts change]
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>