target/mips: Add bit encoding for MXU operand getting pattern 'optn2'

Add bit encoding for MXU operand getting pattern 'optn2'.
Reviewed-by: NStefan Markovic <smarkovic@wavecomp.com>
Signed-off-by: NCraig Janeczek <jancraig@amazon.com>
Signed-off-by: NAleksandar Markovic <amarkovic@wavecomp.com>

target/mips/translate.c

@@ -23995,6 +23995,12 @@ static void decode_opc_special(CPUMIPSState *env, DisasContext *ctx)
 #define MXU_EPTN2_SA    2
 #define MXU_EPTN2_SS    3
 
+/* MXU operand getting pattern 'optn2' */
+#define MXU_OPTN2_WW    0
+#define MXU_OPTN2_LW    1
+#define MXU_OPTN2_HW    2
+#define MXU_OPTN2_XW    3
+
 
 /*
  *