dt-bindings: net: remove arc_emac.txt

The last real user nSIM_700 of the "snps,arc-emac" compatible string in
a driver was removed in 2019. The use of this string in the combined DT of
rk3066a/rk3188 as place holder has also been replaced, so
remove arc_emac.txt
Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/arc_emac.txt

-* Synopsys ARC EMAC 10/100 Ethernet driver (EMAC)
-
-Required properties:
-- compatible: Should be "snps,arc-emac"
-- reg: Address and length of the register set for the device
-- interrupts: Should contain the EMAC interrupts
-- max-speed: see ethernet.txt file in the same directory.
-- phy: see ethernet.txt file in the same directory.
-
-Optional properties:
-- phy-reset-gpios : Should specify the gpio for phy reset
-- phy-reset-duration : Reset duration in milliseconds.  Should present
-  only if property "phy-reset-gpios" is available.  Missing the property
-  will have the duration be 1 millisecond.  Numbers greater than 1000 are
-  invalid and 1 millisecond will be used instead.
-
-Clock handling:
-The clock frequency is needed to calculate and set polling period of EMAC.
-It must be provided by one of:
-- clock-frequency: CPU frequency.
-- clocks: reference to the clock supplying the EMAC.
-
-Child nodes of the driver are the individual PHY devices connected to the
-MDIO bus. They must have a "reg" property given the PHY address on the MDIO bus.
-
-Examples:
-
-	ethernet@c0fc2000 {
-		compatible = "snps,arc-emac";
-		reg = <0xc0fc2000 0x3c>;
-		interrupts = <6>;
-		mac-address = [ 00 11 22 33 44 55 ];
-
-		clock-frequency = <80000000>;
-		/* or */
-		clocks = <&emac_clock>;
-
-		max-speed = <100>;
-		phy = <&phy0>;
-
-		#address-cells = <1>;
-		#size-cells = <0>;
-		phy0: ethernet-phy@0 {
-			reg = <1>;
-		};
-	};