net: phy: provide full set of accessor functions to MMD registers

This adds full set of locked and unlocked accessor functions to read and
write PHY MMD registers and/or bitfields.

Set of functions exactly matches what is already available for PHY
legacy registers.

Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>