spi: dt-bindings: mediatek: Set min size for 'mediatek,pad-select'

The minimum array length defaults to the same size as the maximum. For
'mediatek,pad-select', the example has a length of 2 and in-tree .dts
files have a length of 1, but the schema says the length must be 4.

There's currently no warning in the example because the schema fixups
are not handling this case correctly.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220301212358.1887668-1-robh@kernel.org


Signed-off-by: Mark Brown <broonie@kernel.org>