riscv: dts: sifive: convert isa detection to new properties

riscv: dts: sifive: convert isa detection to new properties

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>