spi: spi-rockchip: add support for spi slave mode

Add support for spi slave mode in spi-rockchip. The register map has an entry
for it. If spi-slave is set in dts, set this corresponding bit and add to
mode_bits the SPI_NO_CS, allow slave mode without explicit CS use.
Slave abort function had been added.

Signed-off-by: Chris Ruehl <chris.ruehl@gtsys.com.hk>
Link: https://lore.kernel.org/r/20200511083022.23678-3-chris.ruehl@gtsys.com.hk
Signed-off-by: Mark Brown <broonie@kernel.org>