drivers: ethernet: phy: Implement clause 45 register read/write APIs

Some devices like Microchip's LAN8650/1 has direct clause 45 register
access where clause 22 indirect access is not needed to read/write
clause 45 registers. PHY drivers can implement read_c45/write_c45
functions using mdio_read_c45/mdio_write_c45 if direct access is
supported OR mdio_read/mdio_write indirect approach to access clause 45
registers indirectly.

Signed-off-by: Parthiban Veerasooran <parthiban.veerasooran@microchip.com>