net: stmmac: dwmac-generic: use devm_stmmac_pltfr_probe()

Use the devres variant of stmmac_pltfr_probe() and finally drop the
remove() hook.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>