riscv: hwprobe: Document MIPS xmipsexectl vendor extension

Document support for MIPS vendor extensions using the key
"RISCV_HWPROBE_KEY_VENDOR_EXT_MIPS_0" and xmipsexectl vendor extension
using the key "RISCV_HWPROBE_VENDOR_EXT_XMIPSEXECTL".

Signed-off-by: Aleksa Paunovic <aleksa.paunovic@htecgroup.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20250724-p8700-pause-v5-5-a6cbbe1c3412@htecgroup.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>

Author: aleksapaunovic-htec

Documentation/arch/riscv/hwprobe.rst

@@ -327,6 +327,15 @@ 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_MIPS_0`: A bitmask containing the
+  mips vendor extensions that are compatible with the
+  :c:macro:`RISCV_HWPROBE_BASE_BEHAVIOR_IMA`: base system behavior.
+
+  * MIPS
+
+    * :c:macro:`RISCV_HWPROBE_VENDOR_EXT_XMIPSEXECTL`: The xmipsexectl vendor
+        extension is supported in the MIPS ISA extensions spec.
+
 * :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.