Default Programmable Buffer Length in rk3399's gmac is not suitable
for MTUs higher than 1498. The easiest solution would be to disable
offloading but it is not ideal as it disables hardware checksumming.

This patch sets pbl to 0x4 which is good for the most popular MTU
value of 1500.

diff --git a/arch/arm64/boot/dts/rockchip/rk3399.dtsi b/arch/arm64/boot/dts/rockchip/rk3399.dtsi
index 9870df76..25aa0fc2 100644
--- a/arch/arm64/boot/dts/rockchip/rk3399.dtsi
+++ b/arch/arm64/boot/dts/rockchip/rk3399.dtsi
@@ -292,6 +292,7 @@
 		resets = <&cru SRST_A_GMAC>;
 		reset-names = "stmmaceth";
 		rockchip,grf = <&grf>;
+		snps,pbl = <0x4>;
 		status = "disabled";
 	};