net: sun8i-emac: Lower MDIO frequency

When sending a command via the MDIO bus, the Designware MAC expects some
bits in the CMD register to describe the clock divider value between
the main clock and the MDIO clock.
So far we were omitting these bits, resulting in setting "00", which
means "/ 16", so ending up with an MDIO frequency of either 18.75 or
12.5 MHz.
All the internal PHYs in the H3/H5/H6 SoCs as well as the Gbit Realtek
PHYs seem to be fine with that - although it looks like to be severly
overclocked (the MDIO spec limits the frequency to 2.5 MHz).
However the external 100Mbit PHY on the Pine64 (non-plus) board is
not happy with that, Ethernet was actually never working there, as the
PHY didn't probe.

As we set the EMAC clock (via AHB2) to 300 MHz in ATF (on the 64-bit
SoCs), and use 200 MHz on the H3, we need the highest divider of 128
to let the MDIO clock end up below the required 2.5 MHz.

This enables Ethernet on the Pine64(non-plus).

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Tested-by: Amit Singh Tomar <amittomer25@gmail.com> # Pine64+
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>