dt-bindings: serial: snps-dw-apb-uart: add compatible property for rk3308 uart

dt-bindings: serial: snps-dw-apb-uart: add compatible property for rk3308 uart

A test with the command below gives these errors:

arch/arm64/boot/dts/rockchip/rk3308-evb.dt.yaml: serial@ff0d0000: compatible:
['rockchip,rk3308-uart', 'snps,dw-apb-uart']
is not valid under any of the given schemas
arch/arm64/boot/dts/rockchip/rk3308-roc-cc.dt.yaml: serial@ff0a0000: compatible:
['rockchip,rk3308-uart', 'snps,dw-apb-uart']
is not valid under any of the given schemas

The compatible property for the Rockchip rk3308 uart was somehow never
added to the documention. Fix this error by adding 'rockchip,rk3308-uart'
to snps-dw-apb-uart.yaml.

make ARCH=arm64 dtbs_check
DT_SCHEMA_FILES=Documentation/devicetree/bindings/serial/
snps-dw-apb-uart.yaml

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>