net: dsa: mt7530: improve and relax PHY driver dependency

Different MT7530 variants require different PHY drivers.
Use 'imply' instead of 'select' to relax the dependency on the PHY
driver, and choose the appropriate driver.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>