spi: dt-bindings: xilinx: Drop type reference on *-bits properties

Properties with standard unit suffixes such as '-bits' don't need a
type.

Cc: Mark Brown <broonie@kernel.org>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: linux-spi@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210910165945.2852999-1-robh@kernel.org