net: stmmac: rk: add struct for programming register based speeds

net: stmmac: rk: add struct for programming register based speeds

There is a common pattern in the driver where many SoCs need to write a
single register with a value dependent on the interface mode and speed.
Rather than having a lot of repeated code, add some common functions
and a struct to contain the values to be written to a register to
select the RGMII and RMII speeds.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1uPk2t-004CFN-Td@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>