riscv: dts: renesas: Convert isa detection to new properties

Convert the RZ/Five devicetrees to use the new properties
"riscv,isa-base" & "riscv,isa-extensions".
For compatibility with other projects, "riscv,isa" remains.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20231009-smog-gag-3ba67e68126b@wendy


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>