dt-bindings: net: dwmac-sun8i: Add binding for GMAC on Allwinner R40 SoC

The Allwinner R40 SoC has the EMAC controller supported by dwmac-sun8i.
It is named "GMAC", while EMAC refers to the 10/100 Mbps Ethernet
controller supported by sun4i-emac. The controller is the same, but
the R40 has the glue layer controls in the clock control unit (CCU),
with a reduced RX delay chain, and no TX delay chain.

This patch adds the R40 specific bits to the dwmac-sun8i binding.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>