lantiq: dts: fix erroneous SPI bus warnings

lantiq: dts: fix erroneous SPI bus warnings

dtc has new checks for SPI buses. The lantiq dts files have a node named
spi' which causes false positive warnings. As the node is a pinctrl child
node, change the node name to be 'spi-pins' to fix the warnings.

Add the 'pins' suffix to all pinmux childs having generic names as well,
to be future proof.

arch/mips/boot/dts/lantiq/vr9.dtsi:343.18-353.6: Warning (spi_bus_bridge): /fpi@10000000/pinmux@e100b10/spi: incorrect #address-cells for SPI bus
arch/mips/boot/dts/lantiq/vr9.dtsi:343.18-353.6: Warning (spi_bus_bridge): /fpi@10000000/pinmux@e100b10/spi: incorrect #size-cells for SPI bus

Signed-off-by: Mathias Kresin <dev@kresin.me>