riscv: dts: sifive: fu540-c000: Fix PLIC node

Fix the device node for the Platform-Level Interrupt Controller (PLIC):
  - Add missing "#address-cells" property,
  - Sort properties according to DT bindings.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>