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

Since commit 39a6ac11 ("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: 39a6ac11 ("spi/pl022: Devicetree support w/o platform data")
Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Mark Brown <broonie@kernel.org>