dt-bindings: net: rockchip-dwmac: Fix rockchip,rk3308-gmac compatible

Schema validation using rockchip,rk3308-gmac compatible fails with:

ethernet@ff4e0000: compatible: ['rockchip,rk3308-gmac'] does not contain items matching the given schema
from schema $id: http://devicetree.org/schemas/net/rockchip-dwmac.yaml#
ethernet@ff4e0000: Unevaluated properties are not allowed ('interrupt-names', 'interrupts', 'phy-mode',
'reg', 'reset-names', 'resets', 'snps,reset-active-low', 'snps,reset-delays-us',
'snps,reset-gpio' were unexpected)
from schema $id: http://devicetree.org/schemas/net/rockchip-dwmac.yaml#

Add rockchip,rk3308-gmac to snps,dwmac.yaml to fix DT schema validation.

Fixes: 2cc8c910f515 ("dt-bindings: net: rockchip-dwmac: add rk3308 gmac compatible")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240528093751.3690231-1-jonas@kwiboo.se
Signed-off-by: Jakub Kicinski <kuba@kernel.org>