net: stmmac: provide generic implementation for set_clk_tx_rate method

Provide a generic implementation for the set_clk_tx_rate method
introduced by the previous patch, which is capable of configuring the
MAC transmit clock for 10M, 100M and 1000M speeds for at least MII,
GMII, RGMII and RMII interface modes.

Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1tna0K-0052sY-QF@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>