arm64: dts: rockchip: set num-cs property for spi on px30

The px30 has two spi controllers with two chip-selects each.
The num-cs property is specified as the total number of chip
selects a controllers has and is used since 2020 to find uses
of chipselects outside that range in the Rockchip spi driver.

Without the property set, the default is 1, so spi devices
using the second chipselect will not be created.

Fixes: eb1262e3

 ("spi: spi-rockchip: use num-cs property and ctlr->enable_gpiods")
Signed-off-by: Heiko Stuebner <heiko.stuebner@cherry.de>
Reviewed-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Link: https://lore.kernel.org/r/20240119101656.965744-1-heiko@sntech.de
Signed-off-by: Heiko Stuebner <heiko@sntech.de>