net: phy: meson-gxl: define control registers

Define registers and bits in meson-gxl PHY driver to make a bit
more human friendly. No functional change.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>