net: stmmac: dwmac-meson8b: Fetch the "timing-adjustment" clock

The PRG_ETHERNET registers have a built-in timing adjustment circuit
which can provide the RX delay in RGMII mode. This is driven by an
external (to this IP, but internal to the SoC) clock input. Fetch this
clock as optional (even though it's there on all supported SoCs) since
we just learned about it and existing .dtbs don't specify it.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>