spi: dt-bindings: spi-controller: remove unnecessary 'maxItems: 1' from reg

Mixing array constraints like 'maxItems' and string or integer value
constraints like 'minimum' don't make sense. Also, with only value
constraints, it is implied we have a single value. So lets remove
'maxItems: 1'.

Cc: Mark Brown <broonie@kernel.org>
Cc: linux-spi@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20190709192631.16394-1-robh@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>