net: stmmac: rk: rk3528: gmac0 only supports RMII

net: stmmac: rk: rk3528: gmac0 only supports RMII

RK3528 gmac0 dtsi contains:

                gmac0: ethernet@ffbd0000 {
                        phy-handle = <&rmii0_phy>;
                        phy-mode = "rmii";

                        mdio0: mdio {
                                rmii0_phy: ethernet-phy@2 {
                                        phy-is-integrated;
                                };
                        };
                };

This follows the same pattern as rk3328, where this gmac instance
only supports RMII. Disable RGMII in phylink's supported_interfaces
mask for this gmac instance.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patch.msgid.link/E1vnYy1-00000007hp3-0hKm@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>