macb: bindings doc: add sifive fu540-c000 binding

Add the compatibility string documentation for SiFive FU540-C0000
interface.
On the FU540, this driver also needs to read and write registers in a
management IP block that monitors or drives boundary signals for the
GEMGXL IP block that are not directly mapped to GEMGXL registers.
Therefore, add additional range to "reg" property for SiFive GEMGXL
management IP registers.

Signed-off-by: Yash Shah <yash.shah@sifive.com>
Reviewed-by: Paul Walmsley <paul.walmsley@sifive.com>

Documentation/devicetree/bindings/net/macb.txt

@@ -15,8 +15,11 @@ Required properties:
   Use "atmel,sama5d4-gem" for the GEM IP (10/100) available on Atmel sama5d4 SoCs.
   Use "cdns,zynq-gem" Xilinx Zynq-7xxx SoC.
   Use "cdns,zynqmp-gem" for Zynq Ultrascale+ MPSoC.
+  Use "sifive,fu540-macb" for SiFive FU540-C000 SoC.
   Or the generic form: "cdns,emac".
 - reg: Address and length of the register set for the device
+	For "sifive,fu540-macb", second range is required to specify the
+	address and length of the registers for GEMGXL Management block.
 - interrupts: Should contain macb interrupt
 - phy-mode: See ethernet.txt file in the same directory.
 - clock-names: Tuple listing input clock names.