Rename SPI controller node in the XTFPGA DTS to spi@...
This fixes the following build warnings:

arch/xtensa/boot/dts/kc705_nommu.dtb: Warning (spi_bus_bridge):
 /soc/spi-master@0d0a0000: node name for SPI buses should be 'spi'
arch/xtensa/boot/dts/kc705_nommu.dtb: Warning (spi_bus_reg):
 Failed prerequisite 'spi_bus_bridge'
arch/xtensa/boot/dts/lx200mx.dtb: Warning (spi_bus_bridge):
 /soc/spi-master@0d0a0000: node name for SPI buses should be 'spi'
arch/xtensa/boot/dts/lx200mx.dtb: Warning (spi_bus_reg):
 Failed prerequisite 'spi_bus_bridge'
arch/xtensa/boot/dts/kc705.dtb: Warning (spi_bus_bridge):
 /soc/spi-master@0d0a0000: node name for SPI buses should be 'spi'
arch/xtensa/boot/dts/kc705.dtb: Warning (spi_bus_reg):
 Failed prerequisite 'spi_bus_bridge'
arch/xtensa/boot/dts/ml605.dtb: Warning (spi_bus_bridge):
 /soc/spi-master@0d0a0000: node name for SPI buses should be 'spi'
arch/xtensa/boot/dts/ml605.dtb: Warning (spi_bus_reg):
 Failed prerequisite 'spi_bus_bridge'
arch/xtensa/boot/dts/lx60.dtb: Warning (spi_bus_bridge):
 /soc/spi-master@0d0a0000: node name for SPI buses should be 'spi'
arch/xtensa/boot/dts/lx60.dtb: Warning (spi_bus_reg):
 Failed prerequisite 'spi_bus_bridge'
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>