Documentation: devicetree: net: add NS2 bindings to amac

Clean-up the documentation to the bgmac-amac driver, per suggestion by
Rob Herring, and add details for NS2 support.

Signed-off-by: Jon Mason <[e-mail]>
Reviewed-by: Florian Fainelli <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/devicetree/bindings/net/brcm,amac.txt

···
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 Required properties:
- - compatible:	"brcm,amac" or "brcm,nsp-amac"
- - 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 "amac_base" and
-		"idm_base"
+ - compatible:	"brcm,amac"
+		"brcm,nsp-amac"
+		"brcm,ns2-amac"
+ - reg:		Address and length of the register set for the device. It
+		contains the information of registers in the same order as
+		described by reg-names
+ - reg-names:	Names of the registers.
+		"amac_base":	Address and length of the GMAC registers
+		"idm_base":	Address and length of the GMAC IDM registers
+		"nicpm_base":	Address and length of the NIC Port Manager
+				registers (required for Northstar2)
  - interrupts:	Interrupt number
 
 Optional properties: