spi: rockchip: add compatible string for px30 rk3308 rk3328

The Rockchip spi binding is updated to yaml and new models
were added. The spi on px30,rk3308 and rk3328 are the same as
other Rockchip based SoCs, so add compatible string for it.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Link: https://lore.kernel.org/r/20200309151004.7780-1-jbx6244@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>