net: phy: at803x: move at8035 specific DT parse to dedicated probe

Move at8035 specific DT parse for clock out frequency to dedicated probe
to make at803x probe function more generic.

This is to tidy code and no behaviour change are intended.

Detection logic is changed, we check if the clk 25m mask is set and if
it's not zero, we assume the qca,clk-out-frequency property is set.

The property is checked in the generic at803x_parse_dt called by
at803x_probe.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>