net: stmmac: rk: add get_interfaces() implementation

net: stmmac: rk: add get_interfaces() implementation

RK platforms support RGMII and/or RMII depending on the SoC. Detect
whether support for a SoC exists by whether the interface specific
set_to functions have been populated, and set the appropriate bits in
phylink's bitmap of interfaces.

This assumes all dwmac interfaces on a SoC have identical support,
but it should be noted that this is not true for RK3528 which only
supports RGMII on GMAC1. However, the existing code structure
permits RGMII to be configured on GMAC0 without complaint, so
preserve this behaviour even though it is incorrect to avoid
functional change.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1uPk2j-004CF6-Mf@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>