riscv: hwprobe: Document SiFive xsfvfwmaccqqq vendor extension

Document the support for matrix multiply accumulate instruction
from SiFive using RISCV_HWPROBE_VENDOR_EXT_XSFVFWMACCQQQ.

Signed-off-by: Cyan Yang <cyan.yang@sifive.com>
Link: https://lore.kernel.org/r/20250418053239.4351-12-cyan.yang@sifive.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: cyanyang-sifive

Documentation/arch/riscv/hwprobe.rst

@@ -353,3 +353,7 @@ The following keys are defined:
     * :c:macro:`RISCV_HWPROBE_VENDOR_EXT_XSFVFNRCLIPXFQF`: The Xsfvfnrclipxfqf
         vendor extension is supported in version 1.0 of SiFive FP32-to-int8 Ranged
 	Clip Instructions Extensions Specification.
+
+    * :c:macro:`RISCV_HWPROBE_VENDOR_EXT_XSFVFWMACCQQQ`: The Xsfvfwmaccqqq
+        vendor extension is supported in version 1.0 of Matrix Multiply Accumulate
+	Instruction Extensions Specification.