arm64: dts: rockchip: Add txpbl node for RK3399/RK3328

Some rockchip SoCs like the RK3399 and RK3328 exhibit an issue
where tx checksumming does not work with packets larger than 1498.

The default Programmable Buffer Length for TX in these GMAC's is
not suitable for MTUs higher than 1498. The workaround is to disable
TX offloading with 'ethtool -K eth0 tx off rx off' causing performance
impacts as it disables hardware checksumming.

This patch sets snps,txpbl to 0x4 which is a safe number tested ok for
the most popular MTU value of 1500.

For reference, see https://lkml.org/lkml/2019/4/1/1382.
Signed-off-by: Carlos de Paula <me@carlosedp.com>
Link: https://lore.kernel.org/r/20200218221040.10955-1-me@carlosedp.comSigned-off-by: Heiko Stuebner <heiko@sntech.de>

arch/arm64/boot/dts/rockchip/rk3328.dtsi

@@ -906,6 +906,7 @@ gmac2io: ethernet@ff540000 {
 		resets = <&cru SRST_GMAC2IO_A>;
 		reset-names = "stmmaceth";
 		rockchip,grf = <&grf>;
+		snps,txpbl = <0x4>;
 		status = "disabled";
 	};
 
@@ -927,6 +928,7 @@ gmac2phy: ethernet@ff550000 {
 		reset-names = "stmmaceth", "mac-phy";
 		phy-mode = "rmii";
 		phy-handle = <&phy>;
+		snps,txpbl = <0x4>;
 		status = "disabled";
 
 		mdio {

arch/arm64/boot/dts/rockchip/rk3399.dtsi

@@ -288,6 +288,7 @@ gmac: ethernet@fe300000 {
 		resets = <&cru SRST_A_GMAC>;
 		reset-names = "stmmaceth";
 		rockchip,grf = <&grf>;
+		snps,txpbl = <0x4>;
 		status = "disabled";
 	};