Convert the fu540 and fu740 devicetrees to use the new properties "riscv,isa-base" & "riscv,isa-extensions". For compatibility with other projects, "riscv,isa" remains. Reviewed-by: Samuel Holland <samuel.holland@sifive.com> Signed-off-by: Conor Dooley <conor.dooley@microchip.com>