dt-bindings: net: Remove duplicate NSP Ethernet MAC binding document

Commit 07d4510f ("dt-bindings: net: bgmac: add bindings documentation for
bgmac") added both brcm,amac-nsp.txt and brcm,bgmac-nsp.txt. The former is
actually the one that got updated and is in use by the bgmac driver while the
latter is duplicating the former and is not used nor updated.

Fixes: 07d4510f ("dt-bindings: net: bgmac: add bindings documentation for bgmac")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/brcm,bgmac-nsp.txt

-Broadcom GMAC Ethernet Controller Device Tree Bindings
--------------------------------------------------------------
-
-Required properties:
- - compatible:	"brcm,bgmac-nsp"
- - reg:		Address and length of the GMAC registers,
-		Address and length of the GMAC IDM registers
- - reg-names:	Names of the registers.  Must have both "gmac_base" and
-		"idm_base"
- - interrupts:	Interrupt number
-
-Optional properties:
-- mac-address:	See ethernet.txt file in the same directory
-
-Examples:
-
-gmac0: ethernet@18022000 {
-	compatible = "brcm,bgmac-nsp";
-	reg = <0x18022000 0x1000>,
-	      <0x18110000 0x1000>;
-	reg-names = "gmac_base", "idm_base";
-	interrupts = <GIC_SPI 147 IRQ_TYPE_LEVEL_HIGH>;
-	status = "disabled";
-};