riscv: dts: Add DT node for SiFive FU540 Ethernet controller driver

DT node for SiFive FU540-C000 GEMGXL Ethernet controller driver added

Signed-off-by: Yash Shah <yash.shah@sifive.com>
Reviewed-by: Sagar Kadam <sagar.kadam@sifive.com>
Cc: Andrew Lunn <andrew@lunn.ch>
[paul.walmsley@sifive.com: changed "phy1" to "phy0" at Andrew Lunn's
 suggestion]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>