sun8i_emac: add support for setting EMAC TX/RX delay

sun8i_emac: add support for setting EMAC TX/RX delay

Some boards have the EMAC TX/RX lanes wired with a different length with
the clock lane, which can be workarounded by setting a TX/RX delay in
the EMAC.

This kind of delays are already defined in the newest device tree
binding of dwmac-sun8i, which has already entered linux-next.

Add support for setting these delays.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Jagan Teki <jagan@openedev.com>