dt-bindings: riscv: Add xtheadvector ISA extension description

dt-bindings: riscv: Add xtheadvector ISA extension description

The xtheadvector ISA extension is described on the T-Head extension spec
Github page [1] at commit 95358cb2cca9.

Link: https://github.com/T-head-Semi/thead-extension-spec/blob/95358cb2cca9489361c61d335e03d3134b14133f/xtheadvector.adoc
Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Tested-by: Yangyu Chen <cyy@cyyself.name>
Link: https://lore.kernel.org/r/20241113-xtheadvector-v11-1-236c22791ef9@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>