phy: lynx-28g: convert iowrite32() calls with magic values to macros

commit  134cd5b20205bd763f8cf19ca02254537baa9daa from
https://github.com/nxp-imx/linux-imx

 lf-6.1.y

The driver will need to become more careful with the values it writes to
the TX and RX equalization registers. As a preliminary step, convert the
magic numbers to macros defining the register field meanings.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>