ASoC: rockchip-spdif: add description for rk3308

A test with the command below shows that the compatible string

"rockchip,rk3308-spdif", "rockchip,rk3328-spdif"

is already in use, but is not added to a document.
The current fallback string "rockchip,rk3328-spdif" points to a data
set enum RK_SPDIF_RK3366 in rockchip_spdif.c that is not used both
in the mainline as in the manufacturer kernel.
(Of the enum only RK_SPDIF_RK3288 is used.)
So if the properties don't change we might as well use the first SoC
in line as fallback string and add the description for rk3308 as:

"rockchip,rk3308-spdif", "rockchip,rk3066-spdif"

make ARCH=arm64 dtbs_check
DT_SCHEMA_FILES=Documentation/devicetree/bindings/sound/rockchip-spdif.yaml

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Link: https://lore.kernel.org/r/20200818143727.5882-1-jbx6244@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>