spi: rockchip: add compatible string for rv1108 spi

The spi on rv1108 is the same as other rockchip based
socs, add compatible string for it.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>