spi: pl022: Specify 'num-cs' property as required in devicetree binding

Since commit 39a6ac11df65 ("spi/pl022: Devicetree support w/o platform data")
the 'num-cs' parameter cannot be passed through platform data when probing
with devicetree. Instead, it's a required devicetree property.

Fix the binding documentation so the property is properly specified.

Fixes: 39a6ac11df65 ("spi/pl022: Devicetree support w/o platform data")
Signed-off-by: Ezequiel Garcia <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/spi/spi_pl022.txt

···
 - compatible : "arm,pl022", "arm,primecell"
 - reg : Offset and length of the register set for the device
 - interrupts : Should contain SPI controller interrupt
+- num-cs : total number of chipselects
 
 Optional properties:
-- num-cs : total number of chipselects
 - cs-gpios : should specify GPIOs used for chipselects.
   The gpios will be referred to as reg = <index> in the SPI child nodes.
   If unspecified, a single SPI device without a chip select can be used.