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

Convert the th1520 devicetrees to use the new properties
"riscv,isa-base" & "riscv,isa-extensions".
For compatibility with other projects, "riscv,isa" remains.

Reviewed-by: Jisheng Zhang <jszhang@kernel.org>
Acked-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20231022154135.3746-1-jszhang@kernel.org


Signed-off-by: Arnd Bergmann <arnd@arndb.de>