powerpc/dts: Remove "spidev" nodes

"spidev" is not a real device, but a Linux implementation detail. It has
never been documented either. The kernel has WARNed on the use of it for
over 6 years. Time to remove its usage from the tree.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211217221400.3667133-1-robh@kernel.org