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>

Author: charlie-rivos

Documentation/arch/riscv/hwprobe.rst

@@ -293,3 +293,13 @@ The following keys are defined:
 
   * :c:macro:`RISCV_HWPROBE_MISALIGNED_VECTOR_UNSUPPORTED`: Misaligned vector accesses are
     not supported at all and will generate a misaligned address fault.
+
+* :c:macro:`RISCV_HWPROBE_KEY_VENDOR_EXT_THEAD_0`: A bitmask containing the
+  thead vendor extensions that are compatible with the
+  :c:macro:`RISCV_HWPROBE_BASE_BEHAVIOR_IMA`: base system behavior.
+
+  * T-HEAD
+
+    * :c:macro:`RISCV_HWPROBE_VENDOR_EXT_XTHEADVECTOR`: The xtheadvector vendor
+        extension is supported in the T-Head ISA extensions spec starting from
+	commit a18c801634 ("Add T-Head VECTOR vendor extension. ").