net: stmmac: platform: provide stmmac_pltfr_probe()

Implement stmmac_pltfr_probe() which is the logical API counterpart
for stmmac_pltfr_remove(). It calls the platform's init() callback and
then probes the stmmac device.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Link: https://lore.kernel.org/r/20230623100417.93592-6-brgl@bgdev.pl


Signed-off-by: Jakub Kicinski <kuba@kernel.org>