net: phy: move mmd_phy_read and mmd_phy_write to phylib.h

net: phy: move mmd_phy_read and mmd_phy_write to phylib.h

Helper functions mmd_phy_read and mmd_phy_write are useful for PHYs
which require custom MMD access functions for some but not all MMDs.
Move mmd_phy_read and mmd_phy_write function prototypes from
phylib-internal.h to phylib.h to make them available for PHY drivers.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/79169cd624a3572d426e42c7b13cd2654a35d0cb.1767630451.git.daniel@makrotopia.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>