macb: bindings doc: update sifive fu540-c000 binding

As per the discussion with Nicolas Ferre[0], rename the compatible property
to a more appropriate and specific string.

[0] https://lore.kernel.org/netdev/[e-mail]/

Signed-off-by: Yash Shah <[e-mail]>
Acked-by: Nicolas Ferre <[e-mail]>
Reviewed-by: Paul Walmsley <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/devicetree/bindings/net/macb.txt

···
   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.
+  Use "sifive,fu540-c000-gem" 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
+	For "sifive,fu540-c000-gem", 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.