riscv: hwprobe: Document thead vendor extensions and xtheadvector extension

riscv: hwprobe: Document thead vendor extensions and xtheadvector extension

Document support for thead vendor extensions using the key
RISCV_HWPROBE_KEY_VENDOR_EXT_THEAD_0 and xtheadvector extension using
the key RISCV_HWPROBE_VENDOR_EXT_XTHEADVECTOR.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Evan Green <evan@rivosinc.com>
Tested-by: Yangyu Chen <cyy@cyyself.name>
Link: https://lore.kernel.org/r/20241113-xtheadvector-v11-11-236c22791ef9@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>