doc: phy: Document devm_of_phy_get()

Add the missing documentation for devm_of_phy_get(), which was forgotten
when the function was introduced.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/768d5845668f081620098a0b4479d1481e212bac.1674584626.git.geert+renesas@glider.be


Signed-off-by: Vinod Koul <vkoul@kernel.org>