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: NChen-Yu Tsai <wens@csie.org>
Reviewed-by: NRob Herring <robh@kernel.org>
Acked-by: NMaxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/dwmac-sun8i.txt

@@ -7,6 +7,7 @@ Required properties:
 - compatible: must be one of the following string:
 		"allwinner,sun8i-a83t-emac"
 		"allwinner,sun8i-h3-emac"
+		"allwinner,sun8i-r40-gmac"
 		"allwinner,sun8i-v3s-emac"
 		"allwinner,sun50i-a64-emac"
 - reg: address and length of the register for the device.
@@ -25,8 +26,10 @@ Required properties:
 Optional properties:
 - allwinner,tx-delay-ps: TX clock delay chain value in ps.
 			 Range is 0-700. Default is 0.
+			 Unavailable for allwinner,sun8i-r40-gmac
 - allwinner,rx-delay-ps: RX clock delay chain value in ps.
 			 Range is 0-3100. Default is 0.
+			 Range is 0-700 for allwinner,sun8i-r40-gmac
 Both delay properties need to be a multiple of 100. They control the
 clock delay for external RGMII PHY. They do not apply to the internal
 PHY or external non-RGMII PHYs.