net: phy: realtek: Add tx/rx delay config for 8211e

Some boards need to change the tx/rx delay config in order for
gigabit Ethernet to work.

In Linux commit bbc4d71d6354 ("net: phy: realtek: fix rtl8211e rx/tx
delay config"), Realtek documented the bits for overriding the delays
from the hardware straps.

Copy the logic from linux, so the delay config is set from the PHY's
interface type (the phy-mode property in the device tree).

This removes the need for a one-off workaround for the Pine A64+ board.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>