net: ethernet: ti: am65-cpsw: Convert to devm_of_phy_optional_get()

Use the new devm_of_phy_optional_get() helper instead of open-coding the
same operation.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/01605ea233ff7fc09bb0ea34fc8126af73db83f9.1678280599.git.geert+renesas@glider.be


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