net: phy: meson-gxl: add g12a support

The g12a SoC family uses the type of internal PHY that was used on the
gxl family. The quirks of gxl family, like the LPA register corruption,
appear to have been resolved on this new SoC generation.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>