net: phy: improve rgmii_clock() documentation

net: phy: improve rgmii_clock() documentation

Improve the rgmii_clock() documentation to indicate that it can also
be used for MII, GMII and RMII modes as well as RGMII as the required
clock rates are identical, but note that it won't error out for 1G
speeds for MII and RMII.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1uPjjk-0049pI-MD@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>