riscv: dts: thead: add xtheadvector to the th1520 devicetree

riscv: dts: thead: add xtheadvector to the th1520 devicetree

The th1520 support xtheadvector [1] so it can be included in the
devicetree. Also include vlenb for the cpu. And set vlenb=16 [2].

This can be tested by passing the "mitigations=off" kernel parameter.

Link: https://lore.kernel.org/linux-riscv/20241113-xtheadvector-v11-4-236c22791ef9@rivosinc.com/
Link: https://lore.kernel.org/linux-riscv/aCO44SAoS2kIP61r@ghost/
Signed-off-by: Han Gao <rabenda.cn@gmail.com>
Reviewed-by: Drew Fustini <fustini@kernel.org>
Signed-off-by: Drew Fustini <fustini@kernel.org>