spi: spi-rockchip: use num-cs property and ctlr->enable_gpiods

The original implementation set num_chipselect to ROCKCHIP_SPI_MAX_CS_NUM (2)
which seems wrong here. spi0 has 2 native cs, all others just one. With
enable and use of cs_gpiods / GPIO CS, its correct to set the num_chipselect
from the num-cs property and set max_native_cs with the define.
If num-cs is missing the default set to num_chipselect = 1.

Signed-off-by: Chris Ruehl <chris.ruehl@gtsys.com.hk>
Link: https://lore.kernel.org/r/20200511083022.23678-4-chris.ruehl@gtsys.com.hk
Signed-off-by: Mark Brown <broonie@kernel.org>