net: dsa: mt7530: refactor SGMII PCS creation

Instead of macro templates use a dedidated function and allocated
regmap_config when creating the regmaps for the pcs-mtk-lynxi
instances.
This is in preparation to switching to use unlocked regmap accessors
and have regmap's locking API handle locking for us.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>