net: stmmac: dwmac-sogfpga: use the lynx pcs driver

dwmac_socfpga re-implements support for the TSE PCS, which is identical
to the already existing TSE PCS, which in turn is the same as the Lynx
PCS. Drop the existing TSE re-implemenation and use the Lynx PCS
instead, relying on the regmap-mdio driver to translate MDIO accesses
into mmio accesses.

Add a lynx_pcs reference in the stmmac's internal structure, and use
.mac_select_pcs() to return the relevant PCS to be used.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>