net: phy: dp83826: support TX data voltage tuning

DP83826 offers the possibility to tune the voltage of logical
levels of the MLT-3 encoded TX data. This is useful when there
is a voltage drop in between the PHY and the connector and we
want to increase the voltage levels to compensate for that drop.

Prior to PHY configuration, the driver SW resets the PHY which has
the same effect as the HW reset pin according to the datasheet.
Hence, there's no need to force update the VOD_CFG registers to make
sure they hold their reset values. VOD_CFG registers need to be
updated only if the DT has been configured with values other than
the reset ones.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Catalin Popescu <catalin.popescu@leica-geosystems.com>
Signed-off-by: David S. Miller <davem@davemloft.net>